Kate Stewart

Katherine Sally "Kate" Lethbridge-Stewart (known professionally as Kate Stewart) was the Chief Scientific Officer at UNIT. She was the daughter of Fiona and Alistair Gordon Lethbridge-Stewart and the mother of Gordy and another child.

Kate spent her youth not knowing of her father's activities with extra-terrestrial beings, believing whole-heartedly that his life consisted of a boring and simple military job. She and her mother spent much time ironically joking that he was constantly off on amazing adventures when they both knew the "truth." Kate would not begin to scratch the true surface of her father's life until years later when she shared an encounter against the Great Intelligence with him. She spent some time investigating UNIT and her father's ties to it and soon joined the organisation, rising to its top position and changing it to be a more scientific, peaceful group.

As the leader of UNIT, Kate encountered the Doctor many times during encounters between Earth and alien threats, becoming similar to the Doctor as her father had been. She was at the forefront of the peace treaty between humanity and Zygons on Earth since 2013 and led a human resistance against Sontaran occupation in 2021. In 2022, having begun rebuilding UNIT, she aided the Doctor and her former companions in stopping The Master's Dalek Plan. With her adventures having seemingly granted Kate the status of companion despite only being in the TARDIS once, she joined other former companions of the Doctor in sharing their stories of their experiences.

Childhood
Katherine Sally Lethbridge-Stewart (PROSE: Downtime: Child of the New World) was born to Fiona and Alistair Gordon Lethbridge-Stewart three years into their marriage, which lasted eight years. As a child, her parents never told her of her father's work at UNIT; she was only aware that he was a military man and was under the impression that it was a boring job. She would often pretend that his absences were due to him having amazing adventures rather than the boring military operations that she believed he was involved in. (HOMEVID: Downtime)

Kate often had nightmares about monsters being in her room at night. She once found herself lost in a forest with her father unable to find her. (AUDIO: Narcissus) When she was a little girl, her father was stationed in Edinburgh for a time. (AUDIO: The Enemy Beyond) Kate went to holiday camps in the UK as a child as UNIT wages did not extend to holidays abroad. (AUDIO: Open the Box) When she was five, her parents separated due to her father's long absences, with Fiona taking Kate to live near her parents in Chichester. (PROSE: The Scales of Injustice) According to one account, "Little Katie" once met the Third Doctor. (GAME: Lost in Time)

However she and her mother eventually moved into her Great Uncle Reggie's old, abandoned house in Pinswick. It was while giving a presentation, about her father, at school that she talked about him fighting monsters. This was scoffed at by the teacher and the other children didn't believe her. Later, at home, she and her mother were confronted by Crinix B-Theta and alien spy, disguised as the disbelieving school teacher, looking for information about her father, UNIT and the Doctor. Through a mixture of Fiona, using her uncle's old service revolver, and Kate's persuasion the creature left the house. (PROSE: My Daddy Fights Monsters)

When Kate was seven, she saw the Fourth Doctor and Sarah Jane Smith at the UNIT Research Unit and was confused when her father told her that the Doctor was both a genius and an idiot. (PROSE: The Day of the Doctor) She also met John Benton, who called her "Little Katie". (AUDIO: Call to Arms) She and her father spent some time at Ealsdon House where she befriended Ben Donelly, with whom she would regularly run up and down the long gallery singing a nursery rhyme about a ghost that haunted the place called the Grey Man. (AUDIO: Invocation)

Becoming a mother
Due to her mother blaming the Brigadier for their separation, (PROSE: Of the Future) Kate grew estranged from her father. (HOMEVID: Downtime) By 1989, she was attending a college in Reading, having left home to gain independence, and shared a house with her best friend, Cecilia Hunter. She became attracted to her science teacher, Jonathan James, and, surprised to find that the attraction was mutual, they began dating despite the college's rules.

In December 1989, Kate discovered that she was pregnant and, so as not to ruin Jonathan's career, opted out of college. The couple visited the Brigadier to inform him of the pregnancy but, after witnessing an attempt on her father's life, Kate realised that she could not allow her father to be part of her child's left. She left without having told him of her pregnancy.

Kate and Jonathan lived together on a houseboat moored on a canal in England. (PROSE: Of the Future) Whilst Jonathan wanted to marry Kate, she wanted to remain single and refused him. (PROSE: Downtime) On 25 September, 1990, she gave birth to a son whom she named Gordon James Lethbridge-Stewart, (HOMEVID: Downtime) with "Gordon" coming from her grandfather and "James" from Jonathan. (PROSE: Of the Future)

In 1992, Kate and Jonathan split up, leaving Kate to raise Gordon as a single parent. (PROSE: Downtime) She remained living on the houseboat (PROSE: Of the Future) and saw very little of him after that. (PROSE: Downtime)

Relationship rekindled
In 1995, Kate and Gordon were harassed by students of the New World University, who believed that she might be able to lead them to the Brigadier. Frightened, she reconnected with her father and briefly fought alongside him and Sarah Jane Smith against the New World group. Following the Great Intelligence's defeat, she entered into a friendlier relationship with her father and ensured that he could have a relationship with the grandson whom he had not previously known about. (HOMEVID: Downtime)

Over the years, Kate became close to her nephew, Conall Lethbridge-Stewart, the son of the Brigadier's illegitimate son, Albert Wilson. Conall, only a few months older than Gordon, was extremely close to her son. (PROSE: Acceptance, and then Understanding)

In 1997, Alexander Christian, who was unable to get the Brigadier's number for reasons of security, used Kate in order to get in touch with her father, having easily gotten a hold of her phone number: 0122 69046. (PROSE: The Dying Days)

In October 2003, Kate received a message from Douglas Cavendish and joined him in investigating a haunting. Arriving at Cavendish's isolated cottage, she faced Mastho and discovered that the Sodality had revived and transported him through time for their own purposes. This meant that the threat to the present was neutralised. (HOMEVID: Dæmos Rising) She and Cavendish sought the help of the Brigadier a year later when Gordon and Conall were kidnapped at Gordon's fourteenth birthday party. (PROSE: Downtime: Child of the New World)

By September 2004, Kate and Gordon lived in a house in Streatham, although they often still used the old canal boat. (PROSE: Downtime: Child of the New World) Kate later married and had a second child, but her relationship ended in divorce. (TV: Death in Heaven)

Kate attended university and wrote for the student newspaper. (AUDIO: Invocation)

Reorganising UNIT
Guided by her father and instilled with the mantra that "science leads", a belief for which the Brigadier credited the Doctor, Kate joined UNIT, going by "Kate Stewart" to avoid unwanted favours and so that she would be judged on her own merits. She rose to the position of Chief Scientific Officer and dragged the organisation "kicking and screaming" to her vision of its scientists leading, a process which she did not find especially fun. (TV: The Power of Three)

Kate was given basic medical training on several aliens that UNIT had in cold storage. This did not include Cybermen. (AUDIO: Code Silver)

In November 2011, Kate gave a five-minute speech about duty at Petronella Osgood's first briefing. (PROSE: For the Girl Who Has Everything)

On 16 December, 2011, Kate, Albert and Doris visited the Brigadier in the nursing home and, whilst Albert and Doris were getting hot drinks, she was left alone with her father. He spoke to her about the Doctor, the Laws of Time and when the Doctor might visit. (PROSE: Lucy Wilson and the Bledoe Cadets) Kate, Albert and Doris were present when he died later that day. (PROSE: Acceptance, and then Understanding)

On 24 December, 2011, Kate observed Osgood in the Grey Archive and decided to put the archive on indefinite hiatus, redistributing the team across UNIT and having Osgood answer directly to herself, having been impressed by what she had seen. She sent Osgood home for Christmas and told her that the team would be reassigned in January 2012. (PROSE: For the Girl Who Has Everything)

On 7 January, 2012, Kate attended her father's funeral. She stood next to Lucy Wilson as heartfelt hugs were given. (PROSE: Lucy Wilson and the Bledoe Cadets)

During the Year of the Slow Invasion, Kate freighted Shakri cubes from all over the world for testing and learnt little about them. After detecting a spike of artron energy, Kate met the Eleventh Doctor and, some days later, summoned him and Amy Pond to the Tower of London via his psychic paper. She attempted to warn the governments of the world of the cubes when they started their countdown and, once the incursion was over, she bid farewell to the Doctor, who had worked out that she was the Brigadier's daughter. (TV: The Power of Three)

Stopping a Nestene invasion
Kate invited Osgood to Simon Devlin's demonstration of a 3D printer, where she met with Jacqui McGee who knew who she was. She was concerned that the technology could be used for war applications. (AUDIO: Vanguard)

She later sent Sam Bishop to the Gobi Desert with Osgood to search for a Swarm Leader. She then met with Devlin to find out more information about him to see if he was connected with the Nestene Consciousness. (AUDIO: Earthfall)

Kate asked Osgood to examine the energy unit and the 3D printer, and was later kidnapped by Devlin, who wanted to convert her into an Auton. (AUDIO: Bridgehead)

Bishop sent her to Black Archive 5 to find an item the Ninth Doctor used against the Nestene, anti-plastic. She then went to Devlin Futuretech to buy some time for Osgood and to learn more about the Nestene's plan. (AUDIO: Armageddon)

Tengobushi incident
She got angry when she discovered that Lyme Industries had taken some of UNIT's property under Peter Latcham's orders. She tried to get it back in a covert mission but she discovered that Lyme Industries had taken it all to their headquarters. (AUDIO: Power Cell)

She went to Geneva to get head office to authorise an investigation into the leak in the UK Government and the Tengobushi attack. She got annoyed when Grant Avery called the UK Government to inform them of Kate's actions. After an attack happened on a French mountain near Geneva she helped Avery to investigate. Discovering that it was similar to the attack in London she managed to persuade Avery that she was right. When Avery was captured by the Tengobushi she devised a way for her and Osgood to get inside the ship to aid his escape. She then went back to London to stop Dokan getting into the Tower of London to get the Kamishi energy cell. (AUDIO: Death in Geneva)

She and Osgood had to fly the plane they were on after the Tengobushi killed their pilots. She asked Josh to kidnap Lyme in order to get information on the artefact the company had. She met with Dokan to find out why he wanted the energy cell. She tricked Dokan into meeting her in the Tower and then let a bomb off to destroy the Kamishi energy barrier. (AUDIO: The Battle of the Tower)

She was placed under arrest by Latcham, but was later freed by Osgood and Carter. With the experimental aircraft at the base she went to rendezvous with Bishop in South America, and then travelled to Antarctica to uncover Lyme's generator there. She confronted Lyme and told her the consequences of using alien technology. When the Kamishi arrived to take their technology back, she managed to persuade them not to kill everyone who had knowledge of their technology but use their mind wipe chemicals instead. Back in London she took delight in personally removing Latcham's memory. (AUDIO: Ice Station Alpha)

Investigating the Silence
She decided to up her training of shooting and asked Carter to help her. She put Shindi on undercover observations at Helena Faversham's place to uncover the strange energy readings Osgood was detecting. She was concerned about the conspiracy theorist. She posed as part of the charity commission to talk to Faversham. She tried to negotiate with the Silents but they wouldn't do it. (AUDIO: House of Silents)

She forgot the whole incident with the Silents, though could remember going to Faversham's house. Jacqui got her to remember the investigation and with Osgood and Bishop's help they found the evidence. She found out that the Silents were influencing the UK population to trust Kenneth LeBlanc and that Carter had been brainwashed too. (AUDIO: Square One)

She found it disconcerting about Carter's involvement in the resignation of the PM and that she had to remind her troops about the Silents. When Bishop showed her the moon landing footage with the Silents in them, she realised that it must have been the Doctor who put it there. Kate became suspicious when the Silents got Carter to order a high amount of explosives. She thought that they were planning to assassinate both LeBlanc and the outgoing Prime Minister, as she thought that this was the best way to get media attention and overrule the Doctor's message to kill them all. (AUDIO: Silent Majority)

She was concerned after they lost contact with Osgood and Bishop on the UNIT satellite. She was trying to investigate the cause of the loss of communications. She discovered that the Silents were manipulating the powers of the world to the brink of starting a war with each other. After she realised that the Silence were also provoking the Telokni to fire on the UNIT space station, she used the footage of the destruction cut with the Silents saying they were leaving the Earth, using McGee's editing skills to do it. (AUDIO: In Memory Alone)

Jastrok's revolution
Kate and Osgood inspected a Silurian nest at Scafell per the Earth Reptile Initiative and were attacked by Jastrok, who refused Kate's attempts to enter into a dialogue. They escaped to Mike Yates and John Benton at the White Hart, which Kate had intended to visit the following day, and they were besieged by Silurians. Kate shot and killed Rikala and Ichtess and escaped with the others in Bessie. The base was destroyed, apparently by Captain Burmaster, and Kate invited Yates and Benton to a debriefing in London. (AUDIO: Call to Arms)

Unsure as to whether Project Charybdis was ecologically friendly, Kate tracked down Jo Jones and asked her and Osgood to fly to Ascension Island and inspect the trial run. Kate flew there after contact was lost with Charybdis and rescued Osgood and Jo, after which she prepared to destroy the Sea Devil base. Osgood stopped her by sending Jo to persuade Krelix to return to hibernation, but Burmaster, controlled by Jastrok, freed Krelix's hybrid creatures into the sea. Kate invited Jo to join the debriefing with Yates and Benton and went with Osgood to deal with the creatures. (AUDIO: Tidal Wave)

Kate and Osgood headed to a Silurian research base in Greece, soon to be joined by Sam, and searched for a device to control the sea creatures. Tryska followed and poisoned Kate with her tongue, but Sam gave Kate a broad-range antivenom and she was later able to locate and administer an antitoxin. Before the base was destroyed, she uploaded data from the Silurian systems to the UNIT Cloud and the three of them were airlifted to safety by Josh. (AUDIO: Retrieval)

Still recovering from the venom and unable to get to Great Britain due to the Silurians' siege, Kate passed local control of UNIT to Jo, Yates and Benton due to their experience. She sent Sam to a Lyme Industries weather station to make it snowy and inhospitable to the Silurians, as well as slowing down the sea creatures and allowing the UNIT fleet to return to the UK. She stormed the Houses of Parliament and arrested Jastrok before freezing him and Kalana beneath the Black Archive to await the judgement of their own people. Before she went to speak with Felicity Lyme about fuel for the artifical cold snap, Yates told her that UNIT was in the right hands. (AUDIO: United)

Brief encounters
Using the identity of "Katherine Johnson", Kate travelled to Monte Bosque with Osgood, Josh and Shindi to meet with Maria Gonsalves and investigate the alien artefact that the missing Sam had tracked to her, which turned out to be a Dalek. It tricked Kate and Shindi into going into the jungle to search for a dormant Dalek taskforce, giving it the opportunity to escape and fake its death in the destruction of a tank. When Osgood realised its deception, Kate went with her and Gonsalves to stop it from reassembling itself in the armoury; Gonsalves was killed and the Dalek self-destructed after Josh and Shindi damaged it. Kate left with some knowledge of the Auctioneers and the knowledge that she had survived a Dalek encounter. (AUDIO: The Dalek Transaction)

Noticing an irregularity in UNIT’s paperwork, Kate headed to Ealdon House in Scotland, a research facility set up in her father’s time but abandoned soon after. There she met Ben Donelly, the caretaker, and his mother Alice. Kate’s memories of visiting the house soon returned to her as well as the unsettling nature of the long gallery. Entering there again she had another encounter with the supposed Grey Man and was nearly frightened out of her wits. After recovering, Kate was joined by Osgood and Josh and discovered Alice had discovered a particular resonance in the house that induced fear in the humans. She had wanted to harness this resonance as a weapon before Kate’s father stopped her. With an unknown alien presence drawing near, responding to a message Alice left years ago and hearing of mass outbreaks of terror in the capital, Kate and Osgood deciphered and completed the message, averting the crisis. (AUDIO: Invocation)

Kate observed Shindi's arrival in the Ardennes, via the hologram projector, and his discovery of a Sontaran capsule. Suddenly, they were ambushed by a squad of Sontarans and Kate lost contact with Shindi. Later when he and the Sontarans returned to England, Kate and some UNIT troops met up with them outside the Torrence Research Foundation, having tracked them using the hologram projector. Allying themselves with the Sontarans, UNIT stormed the building and found Osgood held captive. They also uncovered a group of Sontarans cloned by Professor John Torrence, who hoped to sell them off as warriors for hire in the worldwide auction. While shutting the operation down, Kate learned from Christine Colley about the mysterious head of the auction, the Overseer. (AUDIO: The Sontaran Project)

Kate was worried when an experimental travel capsule, obtained from the auctioneers, suddenly vanished from Osgood’s lab taking her and Josh with it. On their return Kate was bemused by how familiar Josh and Osgood had become with each other. She soon realised this Osgood was in fact from a sideways dimension where UNIT was run by a fascist state. With the other Osgood on the run, while armed and dangerous, and the real Osgood returned, Kate was the only one to realise this, preventing Shindi from taking the real Osgood prisoner. Soon the other Osgood returned and attempted to escape again in the capsule. Kate and her team were saved when the capsule managed to disappear just before the dimensional gateway imploded. Kate then informed her team that Sam had more information on the auctioneers and they were to report to her office. (AUDIO: False Negative)

The Auctioneers and the Cyber Invasion
She went with Osgood to find and rescue Bishop from the Auctioneers. At the Auctioneers' facility they were contacted by the Overseer. They then received information that led them to Bristol. She became worried when one of the puzzles involved Heathrow Airport. The Overseer was punishing her for what UNIT had done to their organisation. Kate was warned to keep away from the Auctioneers. (AUDIO: Game Theory)

Kate wanted to get the Auctioneers app pulled from the app stores and deleted from everyone's devices but there was resistance. She monitored Osgood, Shindi and Carter when they went into the virtual world. She was shocked when trying to disconnect the VR equipment from her colleagues. She then went to get more equipment in case she needed to try again. She tried to shoot the Cyberman. (AUDIO: Telepresence)

Kate declared a Code Silver. She then placed Fortress Island under lock-down. Kate knew of the reasons why the Cybermen were created in the first place and it worried her. She initiated a communications blackout to stop the Cybermen. She went to the armoury to get anti-Cybermen weapons that UNIT had developed. She dissected a Cyberman and she thought that the Cyberman's head handles were used to conduct energy. After the Auctioneers app was placed on everyone's phone, she decided to activate the nuclear weapon under the base. (AUDIO: Code Silver)

arrived and disarmed the weapon, requiring some components from the UNIT archives to repair his TARDIS. When the Cybermen took the Master's TARDIS, Kate accepted his "offer" to help defeat the Cybermen. She went with him and Carter back to the UNIT base but her helicopter was hijacked by the Auctioneers. After the Master killed the Auctioneers, she used the weapons held there to destroy the Cybermen that came after her. She watched over the Master whilst her plans were being undertaken, and conversed about her father. She realised that if her plan to destroy the Cyberman failed she would have to kill Osgood. She was slightly worried when the Master became partially Cyber-converted but was glad when he wasn't and wiped the Cybermen out. She pleaded with the Master to stop millions dying, and allowed him to escape once he undid his back-up plan. (AUDIO: Master of Worlds)

A Wirrn incursion and encounters from UNIT's past
She was concerned when the Master left them a parting gift. She later welcomed Shana Siddiqui to UNIT. After the Wirrn escaped she went to Geneva to warn her superiors. She tried to communicate with the Wirrn after they infected Marcus. When the Wirrn eggs hatched she didn't want to blow up the base. She was too late to detonate the base as the Wirrn had disabled the function. The Wirrn wanted to negotiate with her. She asked Shana to discover what had killed the Wirrn in the past. (AUDIO: Hosts of the Wirrn)

She went on a date with Richard which was interrupted by a skyfall incident. She was intrigued at their communication strategy. She wasn't sure that Vara was telling the truth but wanted her to fire a warning shot. She then wanted to listen to the apparent invaders. She was angry that Vara was threatening the lives of the planet. She decided to hand Vara back to her people, though she wasn't sure of this decision. She tried to convince Jarrak not to kill Vara. She rescinded her agreement but Jarrak went on a war footing. (AUDIO: Breach of Trust)

The Ministry of Defence told UNIT that they should use the Pandora Institute as part of their training to make better soldiers. Kate was sceptical of this and went there with Osgood and Carter to investigate. On the island she told Osgood part of this was because of the spate of deaths of people who had visited the institute. She overheard a conversation by its founder Chin Lee and went to find this container one. When she met with Chin Lee, she was told that she wanted to go under treatment but she couldn't remember it. Her biggest fear was to lose her team. She confront Lee about the deaths. She wanted to know what she was doing back in the country. Once the creature was defeated she thought that UNIT should have done more to help Chin Lee after the Master's influence was taken away from her. (AUDIO: Open the Box)

Alien incursions
She took Osgood and Josh to Abbey Marston in order to resurrect Warren Calder and find out where he hid the Silurian plague that he claimed to have stolen shortly before ending his own life. This was revealed to be a ruse by Calder who wanted Kate to resurrect him in Abbey Marston so that he could use the stone circle to steal a new body and cheat a terminal illness that he had been diagnosed with. However, in doing this he released the static, who planned on stealing bodies of their own so that they could exist on Earth. They were defeated by Kate, Osgood, Josh and Calder when they destroyed the stone circle. (AUDIO: This Sleep of Death)

Kate oversaw Osgood and Sam Bishop's investigation of the unusual weather conditions revealed to have been caused by the Animoi. (AUDIO: Tempest)

Temporal anomalies
While investigating temporal anomalies along the Dorset coast, Kate discovered records of a similar incident in 1972, which stated that Jo Grant would give her life to save the world, prompting her to request Jo's assistance. During the investigation, Jo and Kate were pulled through one of the rifts and sent back to 1972, where they met the Third Doctor as he investigated the original anomaly. In an attempt to preserve the timeline, Kate just introduced herself as the head of UNIT in the future, and she and Jo assisted the Doctor in setting up the equipment to shut down the anomalies. While the Doctor and Jo went off to have a meal, Kate was tricked into reprogramming the Doctor's equipment by a voice posing as Osgood from the future. As the Doctor devised a new solution, Kate was convinced to explain the situation to her father over the radio, as the Doctor had worked out her true identity. Although she never revealed her true name, Kate told her father that it was an honour to work with him, and briefly broke down sobbing when her father ended the call by telling her that the future of UNIT was clearly in good hands. After the anomalies were stopped when Jo apparently sacrificed herself, Kate returned to the future, where she and Osgood received a letter from the Doctor which gave them the necessary equations to retrieve Jo from the Time Vortex. (AUDIO: The Sacrifice of Jo Grant)

She investigated her own murder when her body was discovered within a power station. She was suspicious of the director, known as River Song. However the director was in fact a Wampeerix Battle Queen masquerading as River Song, intent on feeding off the energy of the Earth. The real River Song aided Kate and Osgood in defeating the Wampeerix. (AUDIO: The Power of River Song)

Trouble at the National Gallery
UNIT had in its possession a picture of Kate with River Song whilst the latter was wearing her Demons Run garb, as well as a photo of Kate with Clara from her "previous visit". (TV: The Day of the Doctor)

She ordered the airlifting of the TARDIS to London, to the agitation of the Eleventh Doctor. Kate summoned him there under the royal orders of Elizabeth I, giving him a sealed letter from the Queen and proving her credentials by showing him the impossible painting Gallifrey Falls No More within the National Gallery.

The letter decreed that the Doctor be made curator of the Under Gallery and that he be summoned should there be any disturbance there. Kate led the Doctor and Clara Oswald to the scene of the disturbance. There, she showed them more paintings, all landscapes with glass on the floor next to them. When the Doctor jumped through a time fissure and met the Tenth Doctor and the War Doctor. Clara attempted to follow him before Kate restrained her.

As she left the room to request important UNIT files, Kate was accosted by a Zygon who had escaped through one of the paintings and then took her image and pretended to be the real Kate to Clara Oswald. Osgood saved her from the Zygon nest. Kate, along with McGillop and Osgood, confronted their Zygon doppelgangers in the Black Archive. Kate ordered the countdown to detonate the Archive with a nuclear warhead which would not only prevent the Zygons from having control of the many weapons stored there but would also destroy London in the process. The Zygon used Kate's voice to stop the detonation, but Kate countermanded the order.

Kate and her Zygon counterpart bickered over the detonation, before the War, Tenth and Eleventh Doctors triggered the Black Archive's memory modifiers to confuse everybody in the room as to whether they were a Zygon or a human. The two Kates stopped the countdown, and negotiated a peace treaty. (TV: The Day of the Doctor)

Early actions with the Twelfth Doctor
In 2013, UNIT lost one of their lead scientists, John Foster, who died in a car accident. Kate and the team spent the next year trying to reactivate the projects (based off of the technology left by the Cybermen in 2007), but found various codes that blocked access. She visited the home of Foster's family, who were having trouble coping with the accident. It was then that another John Foster broke through into their dimension to meet with his daughter, unbeknownst to them. When London was attacked by a species originating from the void known as the Fractures hunting Foster, Kate aided the Twelfth Doctor and Clara Oswald in pushing back against the aliens. (COMIC: The Fractures)

When the Cybermen emerged from St Paul's Cathedral in the 21st century, Kate arrived with Osgood and UNIT and took and the Twelfth Doctor into custody. She told the Doctor that he was President of Earth and worked with him to battle the Cybermen. When the Cybermen attacked the plane Boat One under Missy's command, part of the fuselage blew off and Kate fell out. She was saved by her father, who had been converted into a Cyberman, and returned safely to the ground. (TV: Death in Heaven)

The Vulpreen invasion
Kate was contacted by Adam Merchant after his team discovered the Arch and sent Josh in to save him when he fell through it. She was suspicious of the Eleven and tracked him down after he escaped, but he had a hypnotised police officer knock her out and took her as a hostage. She was saved after Osgood and Josh invoked the Lionheart Protocol as she anticipated they would and the Eleven was briefly captured before managing to get away. (AUDIO: The Enemy Beyond)

Harry Sullivan called Kate to inform her of the presence of Ice Warriors in the Northern Territory. Leaving Josh in charge, she flew there with Osgood and was forced to land by Jakernia, who later invited the two of them, as well as Harry and Naomi Cross, aboard her spacecraft. Kate contacted UNIT Australia to arrange an evacuation of Alice Springs and helped to contain Abiathar before returning to the United Kingdom. (AUDIO: Fire and Ice)

Kate and Josh sought to stop the Eleven from stealing jewels sold by the Auctioneers, which she eventually realised was part of his plan to build a tracker for the Arch. The two of them were tricked into going to the Caledonian Gallery in Glasgow whilst the Eleven went on an unsuccessful mission to steal the Arch from Middle Mouse. They realised what had happened and flew to Middle Mouse in time to save Jacqui from drowning after the Eleven had Ava Drake throw her into the sea. (AUDIO: Eleven's Eleven)

Kate had the Arch moved to the Under Gallery where the Curator placed it inside of a J. M. W. Turner painting of Hampton Court Palace. When the Eleven learnt of the Arch's location, Kate entered the painting with Osgood, Josh and the Curator and was taken hostage by the Eleven before the Curator defeated him in a swordfight. Ultimately, Jacqui fell through the Arch and it was damaged by the disruptor's explosion on the Void World. (AUDIO: The Curator's Gambit)

With the Eleven prisoner, Kate regularly asked him to help save Jacqui but refused to make any sort of deal with him. She attended the Starseeker conference where she met Hasper and Jimmy Tan and, learning from Hasper that the Eleven was the only one who could repair it, she allowed the Eleven to work on the Arch under guard. The Eleven managed to escape with the Arch after Hasper tried to take them both to Gallifrey, but Jacqui was returned and Kate invited Jimmy to join UNIT. (AUDIO: The Man From Gallifrey)

Kate and Harry went undercover as journalists to investigate Portstone New Town after he and Naomi recognised Director Prast as a Dominator. They learnt of Prast's plan to destroy Earth and, after escaping from the Quarks by forcing them to drain their energy, Kate destroyed the Dominators' only way to get off of Earth and had Prast and Engineer Sabo arrested whilst the Quarks were disassembled and taken to the Black Archive. (AUDIO: Power of the Dominators)

When temporal anomalies appeared in Woolwich, Kate suspected that the Eleven was responsible and, after Osgood and Jacqui disappeared through a time breach, she and Josh followed and arrived in 1997. There, they met Brigadier Winifred Bambera, whom Kate had heard of from her father, and laid a trap for the Eleven in the 2010s. Upon returning to the present, Kate learnt that Jacqui had been compromised by the Vulpreen and had her taken away. Kate was taken through the Arch by a Vulpreen Commander. (AUDIO: The War Factory)

Lord Varliss sent Kate through a prototype Arch on the Void World where she was enslaved and regularly interrogated regarding Earth's defences. With time moving slower for her, she used the months that she spent there trying to get Worrall and the other slaves to rise up against Varliss. After Varliss killed Worrall, the slaves rebelled and Kate was able to return to Earth with Osgood and Josh, but the Arch's destruction by a bombstrike meant that the Vulpreen were able to escape their prison. (AUDIO: Ten Minutes in Hell)

With a Vulpreen ship heading to Earth, Kate visited Jacqui in a UNIT safe house in Tromsø and learnt the true extent of the Vulpreen's effects on her. She gave her blessing for Jacqui to contact Varliss, but he ordered her to kill Kate and she was forced to return her to normal by shocking her with an exposed wire. An arch arrived nearby in a meteor and Osgood and Jimmy returned from the Vulpreen ship, followed by Vulpreen intent on killing Kate. (AUDIO: The Vulpreen Encounter)

Kate fled from the Vulpreen and was rescued by Naomi. She was contacted by Commander Krillen, who offered to exchange Jacqui for her, and accessed Jacqui's recordings on the UNIT Cloud to find out where she was. She and Jimmy surprised Krillen in Beijing but failed to save Jacqui, who sacrificed herself with a time grenade to kill Krillen and try to destroy the arch. After Jacqui died in her arms, Kate watched as the Axons emerged. (AUDIO: By Jacqui McGee)

Kate and Jimmy were taken to Axos and managed to get free from their restraints thanks to Kate remembering her father's story about Bill Filer and using her heal on Axos's nerve centre. When Yang Li joined her, she sent her to get the injured Jimmy to safety and confronted Axos, tricking them into replicating the Eleven and creating a duplicate for each of his incarnations. Osgood saved her and they returned to Earth where, after a small memorial for Jacqui, Kate received a message from Varliss as the Vulpreen invasion of Earth began. (AUDIO: Axos Unleashed)

To keep Varliss from sending more arches to Earth, Kate travelled through an arch to the asteroid ship with Josh and Jimmy and planted explosives. The timer broke and she volunteered to stay behind the detonate them, but Josh drew the short straw and remained behind. She returned to Earth and fled from Varliss with the help of Osgood and Josh, whose place Daniel Hoyer took, and left the Isle of Wight in a helicopter. (AUDIO: Time of the Vulpreen)

Later actions with the Twelfth Doctor
After the Hyperions conquered Earth, Kate led the population of London underground until the Doctor arrived, at which point she appointed him the President of Earth. (COMIC: The Hyperion Empire)

When Missy returned and froze all the planes on Earth, Clara came to UNIT and Kate helped her contact Missy. (TV: The Magician's Apprentice)

Kate and UNIT went to investigate a Zygon invasion that broke out after a splinter group decided a peace treaty with humans was unacceptable. She was determined to bomb the rebel Zygon settlement, but the Doctor convinced her that this would only make the situation worse – threatening to radicalise every peaceful Zygon on the planet.

Kate followed a lead to Truth or Consequences in New Mexico to try and find the root of the uprising. Here she encountered the one remaining inhabitant of the town, police sheriff C. Norlander, who explained how the trouble started – the accidental revelation of a young Zygon in true form who "hadn't learnt to preserve his body print" had caused panic, distrust and violence within the human population. As the truth of what had really occurred began to dawn on Kate, she realised that Norlander was, in fact, a Zygon rebel agent. (TV: The Zygon Invasion)

She was then attacked but managed to kill the Zygon, before taking its place to fool the High Command. Kate was present in the Black Archive when the Twelfth Doctor forced her to become an ambassador for the human race in an attempt to negotiate a new peace treaty alongside the Zygon High Command leader Bonnie in which a standoff occurred involving the Osgood Box. However, her memories of the Box were soon erased. (TV: The Zygon Inversion)

Along with the Osgoods, Kate watched Daniel Claremont's speech at Ravenscaur School where the Prime Minister was revealed to be a Sea Devil who launched war on the world from Scotland. Though Kate quickly ordered a UNIT fleet against the Sea Devil warships, the Doctor ordered her to pull the ships back, knowing them to be no match for the Sea Devils. (COMIC: Clara Oswald and the School of Death)

After Jenny's bowship had crashed into the Terrance Dicks Library at St Luke's University, Kate issued a D-notice as UNIT began cleaning up the wreckage. The craft was saturated in Void energy however and all of UNIT wound up possessed by the will of a maddened Type 1 TARDIS, being driven to attack the Doctor before the threat was handled. As the Doctor led Bill and Nardole away, he left Kate to resume clean up operations. (COMIC: The Lost Dimension)

Going dark
By 2017, Kate was aware that Prentis was alien and threatened to expose him in 2017 when he informed her that he would be conceding defeat and permitting UNIT's activities to be wound down. When she returned home, she narrowly survived an assassination attempt and called Osgood, telling her that she was "going dark" before breaking her phone. (TV: Survivors of the Flux) In 2018, UNIT was dealing with budget issues brought on by Brexit. Kate should have been leading the operation to deal with a change in time in the year 1896, but her team were busy elsewhere, meaning that Anne Travers and Bill Bishop were taken out of retirement to deal with it. (PROSE: An Ordinary Man)

By New Year's Day 2019, UNIT operations were still suspended due to financial disputes, and the Thirteenth Doctor was unable to contact Kate about an ongoing Dalek threat. (TV: Resolution) Kate still did not work for UNIT in 2020, (PROSE: The Invasion of London, WC: 14684 UNIT Field Log) yet she had some control over its remnants. (WC: 14684 UNIT Field Log)

Kate attended Sarah Jane Smith's memorial on a "bright, cold Spring day", where she discussed Sarah Jane with other guests, giving Sarah Jane's son Luke "the biggest hug", and helped fight the Jackals of the Backwards Clock to foil the Trickster's revenge plot. (WC: Farewell, Sarah Jane) She and the remnants of UNIT began to deal with the Time Fracture at a former Black Site and, on 8 August 2020, after receiving a list of names who could help with the operation from the Doctor, she sent out a message to one of the individuals. This broke security protocols as she was no longer head of UNIT. (WC: 14684 UNIT Field Log)

In 2021, Kate led the human resistance against the Sontarans, hiding in the Williamson Tunnels and in possession of the Doctor's TARDIS. She met the Thirteenth Doctor, Yasmin Khan, Dan Lewis, Eustacius Jericho and Joseph Williamson and brought them up to speed with the occupation, informing them of their presence in Chile and their fondness for chocolate. When Prentis came looking for her, she and Inston-Vee Vinder exiled him through a portal and, following the Sontarans' defeat, she said goodbye to the Doctor and said that she hoped that she would see her again. (TV: The Vanquishers)

In 2022, with memory of the Doctor and the people who'd had contact with them being erased by a psychic virus, (AUDIO: Salvation) Kate led "what's left" of UNIT to try and find her, even though Kate herself could not remember the Doctor. She witnessed Roberts pull a gun on Cleo, Abby and Shawna and tried to defuse the situation, though Cleo's unwillingness to provide information that Kate and Roberts believed she knew made the situation worse. (AUDIO: Interrogation) Kate subsequently disappeared, requiring Osgood to takeover as UNIT's Chief Scientific Officer. (AUDIO: Recruits) The Doctor and Cleo eventually restored the disappeared and stopped the virus. (AUDIO: Salvation)

Stopping the Spy Master
By the time of the The Master's Dalek Plan, Kate had begun rebuilding UNIT. Kate eventually surrendered herself to Ashad in an effort to buy the Doctor some time and she was nearly cyber-converted. However, Kate was rescued by Tegan Jovanka who electrocuted the Cybermen and stopped the conversion process. Escaping from the collapsing building, Kate was invited aboard the Doctor's TARDIS for the first time and was stunned by it being bigger on the inside. Kate helped Tegan, Ace, Graham O'Brien and Yasmin Khan pilot the TARDIS on Qurunx and was returned to Croydon by Yaz afterwards.

Kate was later invited to a support group of the Doctor's former companions alongside Tegan, Ace, Graham, Yaz, Melanie Bush, Jo Jones and Ian Chesterton. Kate joked that she might want to recruit some of them for UNIT, causing everyone to laugh. (TV: The Power of the Doctor)

Personality
Kate disliked the use of weapons, refusing to take a gun from Sarah Jane Smith when offered, (HOMEVID: Downtime) though by the time of the 2010s and 2020s she would, when necessary, use a gun. (TV: The Zygon Inversion, The Vanquishers, et al.)

When she took over UNIT, she enacted great changes to favour scientific approaches over military means. (TV: The Power of Three)

Kate believed that "science leads", which was advice from her late father, (TV: The Power of Three) though "science needs to remember to carry a gun sometimes." (AUDIO: House of Silents)

She believed herself and Osgood to be UNIT's greatest scientific minds. (AUDIO: Retrieval) She usually let Osgood use the call sign Greyhound Two. (AUDIO: Call to Arms)

She refused to treat her people as pawns, but when dealing with the Master, recognised that it was more important to define deaths based on the loss of practical resources and potential allies as appealing to the Master on moral grounds would be pointless. (AUDIO: Master of Worlds)

Kate disliked Elizabeth II's name being used as a nickname for the Wirrn Queen. (AUDIO: Hosts of the Wirrn)

Appearance
Kate had ice-blonde hair and features sharpened by intellect and alertness. (PROSE: For the Girl Who Has Everything) Clara Oswald found her attractive. (PROSE: The Day of the Doctor)

Behind the scenes

 * The character of Kate Stewart was created by Marc Platt for the 1995 direct-to-video story, Downtime. Beverley Cressman portrayed the character. A younger version of Kate appeared in the 1996 Virgin Missing Adventures novel The Scales of Injustice by Gary Russell. She would reappear, again played by Cressman in Dæmos Rising. In 2012 she appeared in the Doctor Who television story The Power of Three, almost twenty years after the character was created. Kate's appearance in The Power of Three marked the first time that a character created for an independent spin-off production appeared in the main series, and followed the on-screen debut of Prof Arthur Candy who originated in the spin-off short story, Continuity Errors.
 * Kate Lethbridge-Stewart was originally planned to appear in the home video Auton with her father and Cavendish. However, Nick Courtney had to drop out of Auton due to a bout of depression and this led to Kate being written out (causing a falling out between Beverly Cressman and writer Nicholas Briggs at the time). A first draft of Auton 2 also featured Kate.
 * Kate Stewart was intended to appear in The Pyramid at the End of the World, but Jemma Redgrave's commitment to Holby City prevented this. Her role was replaced with the Secretary General (who then died).
 * Jemma Redgrave, who is naturally a brunette, plays the role of Kate with blonde hair matching Beverley Cressman's portrayal of the character.
 * The in-universe article A Brief History of the Lethbridge-Stewarts, available as a download from the Lethbridge-Stewart website, says Kate joined UNIT in 2004, following the events of Daemos Rising, at the behest of her father. Because it is non-narrative, it is not a valid source on this wiki.
 * A brief lowdown on the Sea Devils spells her name as "Kate Stewert"