The Sontaran Stratagem (TV story)

The Sontaran Stratagem was the fourth episode of series 4 of Doctor Who.

It was the first appearance of the Sontarans in the revived series and saw the reappearance of Martha as a recurring guest character following the next two episodes.

Also of note was the casting of the Sontarans. As well as featuring the return of Christopher Ryan in a new role, this episode featured the first Doctor Who appearance of Dan Starkey, seen here as Commander Skorr. Primarily recognised later as Commander Strax, a friend of the Doctor, he would go on to appear in various other roles, several of them also Sontaran, in both the TV series and Big Finish Productions' audio dramas during the years that would follow.

Synopsis
Fifty-two people across the world in eleven different time zones die at the exact same time. The only connection: they all have ATMOS installed in their vehicles. Martha Jones, now a UNIT medic, summons the Tenth Doctor back to modern-day Earth to help figure out why, but an old enemy lies in wait...

Plot
Boy genius Luke Rattigan and his students toss out reporter Jo Nakashima and her notes from the Rattigan Academy. She angrily protests, claiming that her investigations of ATMOS, Rattigan's invention, have led her to believe it is dangerous. An uninterested Rattigan allows her to leave. However, upon returning inside, he contacts his mysterious employer and recommends Jo's "termination". His employer reminds him they don't take orders from "humans", but Rattigan replies that he said "recommend", and the employer agrees to terminate Jo.

Sometime later, Jo is driving in her car, which is fitted with an ATMOS-issue Satnav. She leaves a message for Colonel Mace of UNIT, asking him to check out several recent deaths that have occurred in ATMOS-equipped cars and to pinpoint the times. After crossing a bridge, her ATMOS navigation instructions lead her car to turn right and stop at the bank of a river. Jo is confused, as she told ATMOS to take her to UNIT Headquarters. She's about to exit the vehicle and find her own way, when the car doors lock themselves and the transmission locks itself into the drive position. Jo screams helplessly as the car accelerates towards the bank and goes flying, landing in the river, where it sinks with her inside, causing her to drown.

Meanwhile, Donna is flying the TARDIS with mixed results while the Tenth Doctor watches closely, warning her not to get too close to the 1980s. Donna sarcastically wonders if she's going to put a dent in them. ("Well, someone did," the Doctor mutters to himself) He receives a call from a mobile phone on the console, belonging to former companion Martha Jones. When he answers, Martha greets him happily on the other end of the line and tells him that she's bringing him back to Earth.

The TARDIS materialises in an alley, where Martha is waiting. She and the Doctor joyfully reunite, although he grows trepidatious when he introduces Martha to Donna, expecting them to start fighting. Martha and Donna, however, get along almost immediately by making jokes at the Doctor's expense. Donna is the first to notice Martha is wearing a ring on her finger and congratulates Martha, who explains she's now engaged to Tom Milligan and, thanks to the Doctor's recommendation, has a job with UNIT as a medical officer. Their interlude is interrupted when Martha receives a call on a walkie-talkie and gives the all-clear to execute "Operation Blue Sky." Nearby, UNIT forces raid an ATMOS factory and arrest the workers. Unbeknownst to them, Luke Rattigan's mysterious employer is monitoring the raid from afar, and ridicules the soldiers' primitive fabrics and weapons.

Martha leads the Doctor and Donna into a lorry containing a mobile UNIT headquarters, and introduces the Doctor to Colonel Mace, who knows all about the Time Lord from UNIT files. The Doctor isn't impressed with the overly military "new" UNIT, while Donna, already unsettled by how comfortable Martha seems in her new role among soldiers, compares UNIT's handling of civilian workers in the ATMOS factory to Guantanamo Bay.

Martha and Mace explain the reason behind the raid: just the day before, fifty-two people were simultaneously poisoned to death in their cars across the world. Martha has checked the biopsies and found the unknown toxin had left the victims' bodies immediately. The cars in question are of different makes and models, but are all fitted with ATMOS - which stands for ATMospheric Omission System. Donna, who has heard of the system before, aids Martha and Mace in explaining to the Doctor that the new technology reduces carbon emissions from vehicle exhaust fumes to zero. UNIT suspects that such an incredible feat of engineering is alien in origin, and the Doctor theorises that the environmentally-friendly technology could be concealing a weapon, with the 800 million cars on Earth as its arsenal.

Meanwhile, two UNIT soldiers, Privates Harris and Gray, are exploring the lower corridors of the ATMOS factory when they find two ATMOS workers guarding a door. Rattigan's secret employer remotely orders the two workers, who seem to be in a hypnotic trance, to allow the soldiers entry. The two soldiers enter a mysteriously-lit chamber with an armoured vat and hear something alive banging underneath. While the nervous Gray attempts to call for back-up, the more self-assured and inquisitive Harris opens the container to find it is filled with a bubbling green chemical solution. Submerged within is a featureless human shape with an umbilical cord running down its back; it reacts to their presence but shows no signs of aggression.

As Harris figures it must be a cloning tank and someone is growing a body, a squat figure in a domed helmet and full body armour appears in the room and praises Harris's skills of deduction, while belittling Gray's overly-cautious instincts as "smell[ing] of sweat and fear". The arrogant Harris repeatedly insults the warrior's height, provoking him into shooting a bolt of energy from a staff which disables the private's legs. Gray is then goaded into firing his submachine gun, which only clicks as the warrior reveals the room is contained within a cordolaine signal that excites the copper surface of the bullet and causes expansion in the barrel, rendering the soldiers' guns useless, as well as blocking their radio signals. In desperation, Gray raises his gun to physically attack the figure, who retaliates by paralysing him in turn and motions the two ATMOS workers inside to prepare the two UNIT privates for his stratagem. When asked for a name, he identifies himself as General Staal of the Tenth Sontaran Battle Fleet, known as "Staal the Undefeated", and finally removes his helmet to reveal a slimy brown face underneath which horrifies the incapacitated privates.

The Doctor examines one of the ATMOS devices under Martha's watchful eye and concludes it is advanced but not alien technology. His dismissive attitude toward Mace's military background prompts Martha to defend the colonel; after leaving the Doctor, she has tried to use his influence as a force for good and is working with UNIT on her own terms. Donna, meanwhile, has been using her own temp skills to go over the personnel records. She returns with the Sick Days file, which is empty: no employees at the factory have called in sick for months, if ever, which is even more suspicious.

The Doctor and Mace discuss Luke Rattigan, the renowned creator of the ATMOS system who now runs the Rattigan Academy outside London as a school for the exceptionally gifted. The Doctor decides to pay Rattigan a visit and Mace assigns him an escort, private Ross Jenkins. Meanwhile, Donna and Martha have a chance to chat, and Donna is left profoundly concerned when Martha warns her that life with the Doctor can have devastating consequences, and that her own family suffered because she didn't tell them about her involvement with the Doctor.

In the factory basement, Staal bestows the same hypnotic programming on Gray and Harris as the ATMOS workers, assigns them orders and then returns to the Sontarans' ship via a nearby teleport pod. As the Doctor prepares to head to the Rattigan Academy with Jenkins in a jeep, he is approached by Donna who tells him that she's going home. Thinking that she is leaving for good, the Doctor laments the many places they could have travelled together and thanking her for saving his life in more ways than one, until he figures out she only intends to pay a brief visit to her mother and granddad, and is roundly mocked by Donna for his misinterpretation.

Acting on Colonel Mace's orders, Martha speaks to a Polish factory worker named Trepper and finds he is little more than an automaton with an alarmingly rapid heartbeat as he tells her in an emotionless monotone that he came over from Poland just to work, and that he works 24 hours a day. A disturbed Martha goes to report her findings to Mace, when she is intercepted by privates Harris and Gray. Under the pretence of escorting her to the colonel, the two take her down to the basement and into the cloning chamber. Martha realises too late she has been led into a trap, and screams as the brainwashed UNIT soldiers restrain her inside.

The Doctor and Jenkins drop Donna off in Chiswick, and she thinks back on her recent adventures as she walks down her street to her home, where her grandfather Wilfred is waiting with open arms. In the Nobles' kitchen, Donna tells Wilf about her travels; she learns that it's been a few days since the Adipose incident. Her grandfather takes it well, despite some concern for her safety, but warns her not to tell her mother, Sylvia. When Sylvia comes in, she mocks Donna's claims of travelling as "gallivanting around" and scolds her for the trick with the keys; which she did find after all.

On their way to the Rattigan Academy, the Doctor notes that even the UNIT jeeps are equipped with ATMOS. Jenkins claims that UNIT has been watching Rattigan for some time but until his system is proven to be dangerous, the government refuses to remove ATMOS from vehicles. They arrive at the academy to find Luke waiting for them. Despite the "Hitler Youth"-style training regime his students are subjected to, the Doctor is excited by the science experiments being conducted inside the labs, all of which seem centered on terraforming a planet. The insecure Luke feels threatened by the Time Lord's intellect and although he sympathises with the difficulties of being a genius, the Doctor notes that the young man couldn't have created such advanced equipment - such as the large piece of tunnel-like sculpture he keeps in his reception area.

The Doctor triggers a control on the piece of "art", revealing it as a teleport pod, and finds himself transported to the war room onboard the Sontaran spaceship. He teleports back when they notice him, but General Staal follows in pursuit and makes it through before the Doctor sabotages the pod with his sonic screwdriver. By displaying a level of understanding about Sontarans far beyond that of humans, the Doctor prompts Staal into removing his helmet and ridicules his honour. Realising who he's up against, the Time Lord uses a racquet to bounce a squash ball off the teleport pod and hit Staal's probic vent, the Sontarans' one weakness. A stunned Staal collapses, enabling the Doctor and Jenkins to make their escape while he recovers. Rather than pursue them, Staal uses his energy rod to repair the teleporter and returns to his ship with Luke. With 400 million cars converted with ATMOS devices, he has his second-in-command Commander Skorr, "the Bloodbringer", begin the invasion.

Skorr teleports down to the factory basement and confronts a captured Martha, for whom the Sontarans have prepared something more sophisticated than the hypnotic programming they used on the factory drones and the privates. Sure enough, to Martha's horror, the humanoid creature inside the vat emerges from the liquid as a naked replica of herself. Ignoring her protests, Skorr lulls Martha into an unconscious state and begins transferring her memories into the clone.

Luke suggests using the ATMOS to kill the Doctor, confusing Staal, as Luke had previously told him the Doctor hadn't given his name. Staal recalls the legends of the Last Great Time War, which claim the Doctor, led the Time Lords into battle, and becomes elated at the idea of wiping out an old enemy of the Sontarans alongside Earth. As the Doctor and Jenkins begin to return to the ATMOS factory, the Sontarans track their progress through the ATMOS SatNav device in their UNIT jeep and steer it into the river. Finding all the doors deadlock-sealed, the Doctor manages to override the device by reverse psychology: he instructs it to do what it's doing anyway so it will disregard his orders and do the opposite. It stops just short of the river and allows the Doctor and Jenkins to jump out. They dive for cover, expecting an explosion, which turns out to be anticlimactic as the ATMOS just sparks and shorts out.

With the ATMOS signal gone, Luke and Staal assume that the Doctor is dead. Luke assures the general that the initial test run resulting in the fifty-two dead car drivers was successful, and they are ready to unleash the ATMOS devices on a wider scale. Back on Earth, Skorr briefs the Martha clone, who has stripped the original of her clothes and engagement ring to maintain the disguise, and sends it out with the brainwashed Harris and Gray to sabotage UNIT from the inside.

The Doctor reunites with Donna at her home in Chiswick while Jenkins goes to find a vehicle without ATMOS so they can get back to UNIT headquarters. As the Doctor and Donna inspect the ATMOS device on the Nobles' car, they are joined in the driveway by Wilfred, who recognises the Doctor as the same man who disappeared in front of him on Christmas Eve. However, he takes this and the knowledge the Doctor is an alien in his stride, and the two introduce themselves properly. Sylvia, however, is less than impressed to recognise him as the same person who caused so much trouble at Donna's aborted wedding.

The Doctor uses Donna's mobile to call UNIT and speaks with Martha, unaware he is talking to her clone. She promises to pass on his warning about Sontaran invasion, but after ringing off, assures Mace that there is no cause for alarm. The Doctor then resumes his examination of the ATMOS in Donna's car, only to trigger a hidden device exposing gas nozzles. The Sontaran lieutenant, Skree, detects the converter's activation aboard the Sontaran ship, leading Staal to realise the Doctor is still alive. He is delighted, looking forward to a glorious battle, and orders the invasion to begin and the Sontaran ships close in.

The Doctor realises the gas fumes coming from the car are poisonous and Wilfred tries to move it off the street, but at the same time, Staal orders all the ATMOS converters to be activated and every one deadlocked. As Wilfred gets in the car, the doors suddenly lock themselves and the engine starts on its own. Inside, Wilf becomes engulfed in the poisonous gas as the Doctor and Donna try to disable the ATMOS to get him out. All over the world, cars begin emitting the toxic gas into the atmosphere, with their owners unable to stop them. At the factory, the UNIT soldiers try to destroy their ATMOS-fitted cars by shooting at them, while Martha's clone and her Sontaran-controlled escorts look on impassively.

Meanwhile, Wilf begins to choke as the Doctor's desperate efforts to get him out of the car fail. Sylvia hurries back inside the house as car alarms are set off all along the street and the other parked cars begin emitting the gas. The Doctor can only watch helplessly while Donna's entire street is buried in the deadly fog. Up above, General Staal leads Rattigan and his Sontaran troops in a war chant as the invasion begins.

Cast

 * The Doctor - David Tennant
 * Donna Noble - Catherine Tate
 * Martha Jones - Freema Agyeman
 * Wilfred Mott - Bernard Cribbins
 * Sylvia Noble - Jacqueline King
 * Luke Rattigan - Ryan Sampson
 * Colonel Mace - Rupert Holliday Evans
 * General Staal - Christopher Ryan
 * Commander Skorr - Dan Starkey
 * Jo Nakashima - Eleanor Matsuura
 * Private Harris - Clive Standen
 * Private Gray - Wesley Theobald
 * Ross Jenkins - Christian Cooke
 * Worker - Rad Kaim
 * Atmos Voice - Elizabeth Ryder

Uncredited cast

 * Captain Price - Bridget Hodgson
 * Lieutenant Skree - Dan Starkey
 * Sally - Joan Williams
 * ATMOS Workers - Jon Davey, Andrew Slade

Diseases and illnesses

 * Donna is surprised that no ATMOS employees have ever had man flu.

Technology

 * The Sontarans use cloning pools.
 * 400000000 cars have ATMOS devices.

Individuals

 * Martha is now engaged to Tom Milligan.
 * Martha's stated desire to change UNIT from the inside echoes Jack Harkness' similar statement about his intentions regarding Torchwood.
 * Luke Rattigan invented Fountain Six.
 * Donna Noble once dated a boy named Matthew Richards, who in the end turned out to be gay.

Locations

 * The Doctor mentions the fifteenth broken moon of the Medusa Cascade.
 * Streets in Chiswick include Rydale Street and Mallard Street.

Planets

 * The Doctor mentions Cotter Paluni's World as one of the planets he wanted to take Donna.
 * The Doctor also mentions wanting to take Donna to the Diamond Coral Reefs of Kataa Flo Ko.

Species

 * General Staal is called a "baked potato" by Ross Jenkins.
 * The Sontarans' weak point is their probic vent.

Time wars

 * According to Staal, the Sontarans were not allowed to take part in the Last Great Time War.

Timeline

 * The Doctor said he worked with UNIT some time in the 1970s, "or was it the 80s?"

Unified Intelligence Taskforce

 * UNIT has received increased funding from the United Nations.
 * Martha mentions the Valiant.

Story notes

 * The Doctor last saw Martha Jones in the television story Last of the Time Lords. It is not indicated on screen how long it has been for Martha since she last saw the Doctor, although it has been long enough for her to have joined UNIT, obtained her doctorate, become engaged to Tom Milligan and worked with Torchwood. Martha last appeared (chronologically) in A Day in the Death.
 * The Sontarans first appeared in the television serial The Time Warrior and last appeared on a TV broadcast in The Two Doctors, though they did feature in a direct-to-video story, Shakedown: Return of the Sontarans. They also feature in the television stories The Sontaran Experiment and The Invasion of Time. Though physically similar to prior appearances, their uniforms are now blue (though a few Sontarans seen aboard their command ship are seen in armour coloured more like previous versions; however, they still have blue helmets and shoulder pads) and their voices have taken on a more human quality. They are honourable when it comes to the rules of warfare and respect their enemies, but are callous and bloodthirsty.
 * Helen Raynor wrote this episode. Her Series 3 episodes Daleks in Manhattan and Evolution of the Daleks both filled fourth and fifth episode slots in the series. This two-part story also occupies episode four and five.
 * Helen Raynor was originally reluctant to write this story, having read online comments about her previous Dalek two-parter and come away "physically shaking" at the level of vitriol. Russell T Davies convinced her not to take the comments too seriously and to write this story. (REF: The Writer's Tale)
 * This two-parter is Douglas Mackinnon's debut story as the director.
 * When Donna asks the Doctor if it was his influence that turned Martha into "a soldier", a few notes of Donna's theme music can be heard before it segues into the music which plays when the UNIT soldiers appear.
 * Mobile phone company O2 released an ATMOS SatNav phone, possibly based on the devices used in this episode.
 * This show was broadcast during the thirty-fifth anniversary year of the Sontarans.
 * In The Time Warrior the Sontarans were described as "nasty, brutish and short", a reference to Thomas Hobbes. This is the first time that they are shown to actually be shorter than normal human stature.
 * There is a UNIT character called Jenkins, a homage to a famous piece of dialogue: "Jenkins, chap with the wings there Five rounds rapid" from the television story The Dæmons. In the commentary broadcast on BBC7, it was said that "Jenkins" was named for Richard Jenkins, who maintains the BBC's DW website.
 * UNIT has now been rebranded as the Unified Intelligence Taskforce, having previously been known as the United Nations Intelligence Taskforce. This followed a request by the United Nations shortly after the series returned to TV in 2005. Nonetheless, Martha still refers to UNIT receiving funding from the United Nations and the latter Children of Earth: Day One indicates that the UN still commands UNIT.
 * When communicating with each other, UNIT use call signs first used in the classic series. These are mostly based on greyhound racing. Personnel are usually designated a Greyhound call sign and a base of operations is known as a Trap.
 * Donna makes reference to the terrorist prisoner camps at Guantanamo Bay. This is one of the first references in the revived Doctor Who to the aftermath of the 9/11 attacks. Similarly, Martha refers to UNIT's funding being "in the name of homeworld security", a deliberate reference to the Department of Homeland Security established in the US after 9/11.
 * The cloning vat in this story is the fountain from The Fires of Pompeii, redressed. The table that is used to hold the real Martha is used in the Torchwood episode Cyberwoman. Also, the metal headwear that Martha has on at the same time looks like mind probe used the episode Sleeper.
 * This episode, and its follow-up, The Poison Sky, are the only episodes featuring Donna's family to not be written by Russell T Davies.
 * Edward Gomez provided science advice for the writing of this episode.
 * Originally, Geoff Noble was in this episode. Helen Raynor drew upon her own parents' relationship, depicting Geoff being henpecked by Sylvia. However, after filming his scenes for Partners in Crime, it had become clear that Howard Attfield was extremely ill with cancer. After consulting with the actor's wife, Russell T Davies decided to replace him with Wilf. One draft explained that Geoff was unwell.
 * Phil Collinson did not produce this episode. While he took time off, Susie Liggat filled in for him.
 * The UNIT driver Ross Jenkins is a shout-out to script assistant Ross Sutherland and New Media assistant producer Richard Jenkins.
 * Russell T Davies saw the story as an opportunity to finally put UNIT centre-stage. In particular, he was keen to draw a contrast between their approach and the Doctor's more pacifist brand of heroism, because he felt that the Doctor's association with the Taskforce had seemed almost hypocritical in the past.
 * Russell T Davies wanted a key hook for the adventure to be something topical yet seemingly mundane. Originally, this was going to be a new kind of chimney, which would be attached to every home as part of a purported effort to cleanse the atmosphere of pollution. As Helen Raynor developed her narrative, however, it was decided to combine the notion of green technology with the newfound popularity of satellite navigation in automobiles. There had been several recent reports of satnav devices giving drivers incorrect information, leading to accidents and even near-fatalities. It was agreed that modern viewers, especially children, would be better able to relate to a pollution-fighting satnav than a chimney.
 * Initially, cloning was going to be a much more prominent aspect of the story, with the ATMOS factory workers all being clones. Russell T Davies felt that this element would be more effective in small doses, however, which led to the introduction of the cloned Martha.
 * Luke's surname was originally Marlow, in reference to a young fan of Helen Raynor's acquaintance; it was changed to Rattigan, which had been an early name for Miss Foster in Partners in Crime, and had previously been used by Russell T Davies on the soap opera Revelations.
 * The reporter's surname was initially Rosser instead of Nakashima.
 * A long-simmering issue which Russell T Davies decided to confront was UNIT's name. Following its mention in Aliens of London, the BBC was made aware that the real United Nations was now concerned about being associated with a fictional entity, and was especially wary of any confusion which might arise from people encountering online references to UNIT. Davies' initial reaction was simply to avoid any explanation of the UNIT acronym, but he no longer felt that this approach was viable. Having explored a range of options, it was finally agreed that the organisation would now be identified as the Unified Intelligence Taskforce.
 * This two-parter was originally designated Block Four of the recording schedule. However, when Block Three - Partners in Crime and The Fires of Pompeii - was split into two, it became Block Five instead.
 * The task of redesigning the Sontarans fell to Neill Gorton and his team at Millennium FX. Gorton felt that John Friedlander's original facemask design required little modernisation. In greater need of updating was the Sontaran armour, with the quilted texture used in the Seventies now seeming very dated, although both Gorton and Russell T Davies were determined to preserve the dome-like helmets which perfectly matched the shape of the Sontaran head. Recalling that Linx had been mistaken for a knight in The Time Warrior, Gorton strove to give the Sontaran armour a metallic feel.
 * To play the various Sontarans, a number of relatively short-statured artistes were required, only two of whom would be seen without their helmets - Christopher Ryan and Dan Starkey.
 * Dan Starkey was a fresh graduate from drama school when he was cast as Commander Skorr.
 * Rotherham-born Ryan Sampson was twenty-three when he played the eighteen year old American Luke Rattigan.
 * Helen Raynor wrote this episode. Her series 3 episodes Daleks in Manhattan and Evolution of the Daleks both occupied fourth and fifth episode in the series. This two-part story also occupies episode four and five.
 * Helen Raynor was originally reluctant to write this story, having read online comments about her previous Dalek two-parter and come away "physically shaking" at the level of vitriol. Russell T Davies convinced her not to take the comments too seriously and to write this story. (REF: The Writer's Tale)
 * Martha's departure allowed Russell T Davies to change the character's personality. In her reappearance, she is more mature and equal to the Doctor, as opposed to the third series, in which she was in love with him.
 * The writers debated several aspects of Martha's character, her status and reaction to Donna in particular. Helen Raynor elected to emphasise Martha's medical career over her military career, and avoided a "handbags at dawn" scenario because she felt it would rehash Rose Tyler's initial opinion of Sarah Jane Smith in School Reunion.
 * The car going into the river was the first time a car-cannon had been used since 2005, and was required to be completed in one shot. The car fired into the canal was removed immediately afterwards to clear the shipping route.
 * The scenes involving the cloning tank were filmed in a Welsh shampoo factory and reused a prop from The Fires of Pompeii as the tank which contained the clone.
 * Russell T Davies and Freema Agyeman enjoyed the scenes set in the cloning room; Agyeman liked playing an "evil companion", who she and Davies felt made the real Martha "warmer", and Davies thought Privates Gray and Harris discovering the tank in a darkened room was "classic Doctor Who".

Ratings

 * 7.06 million viewers (UK final)

Studio

 * Upper Boat Studios, Trefforest
 * BBC Broadcasting House, Llandaff

Location

 * Margam Country Park, Port Talbot
 * Usk Valley Business Park, Pontypool
 * Nant Fawr Road, Cardiff
 * Orion Electric, Port Talbort
 * Roath Basin, Cardiff Docks

Production errors

 * The UNIT mobile lorry is much bigger on the inside than the outside. This was explained in the next episode's commentary.
 * When the TARDIS lands where the Doctor and Martha are supposed to meet, as the camera pans, you can see that the inside panel of the TARDIS has been removed and the gap attached to the building behind it, to give the TARDIS that "bigger on the inside" look without the use of a green screen.

Continuity

 * Sontarans use hypnosis to control workers and two soldiers, much as another Sontaran Jingo Linx previously did to scientists. (TV: The Time Warrior)
 * Sontarans again make use of Sontaran batons. (TV: The Two Doctors)
 * Donna recalls flashbacks of previous adventures with the Doctor. (TV: Partners in Crime, The Fires of Pompeii, Planet of the Ood)
 * Martha informs Donna that she left the Doctor because of the impact it had on her family, (TV: Last of the Time Lords) as she never truly told them what she was doing. (TV: The Lazarus Experiment, 42)
 * The Doctor uses Martha's phone, which she gave to him when she left. (TV: Last of the Time Lords)
 * From her grandfather and mother's perspective, it has only been days since Donna left with the Doctor. (TV: Partners in Crime)
 * Bea Nelson-Stanley referred to the Sontarans as potato-like. (TV: Eye of the Gorgon) Zorelle called Lieutenant Vorn 'potato-head', angering him and Commander Steg. (HOMEVID: Shakedown: Return of the Sontarans) Clyde Langer would also make a similar remark about "spuds". (TV: The Last Sontaran)
 * Sylvia Noble recognises the Doctor and references Donna's wedding, (TV: The Runaway Bride) while Wilfred Mott recognises the Doctor from his previous brief encounter with him. (TV: Voyage of the Damned) Wilfred's absence from Donna's wedding is explained by Sylvia when she says he was ill with flu at the time.
 * The Doctor's initial response to Martha and Donna meeting each other is to ask them not to fight, which is what happened when Rose Tyler met Sarah Jane Smith. (TV: School Reunion)
 * The Doctor briefly calls Martha's superphone, initially unaware that he is speaking to an impostor when Martha's clone answers. He was previously oblivious to Rose Tyler's possession by Cassandra O'Brien when he first called her on Rose's mobile. (TV: New Earth)
 * The Doctor mentions that the ATMOS devices are a temporal pocket one second out of sync with real time. The same is later said for the Medusa Cascade (TV: The Stolen Earth) and for a time, the Doctor's TARDIS. (TV: The End of Time)
 * Martha's talks of her engagement to Tom Milligan. She met Tom during her last adventure with the Doctor. (TV: Last of the Time Lords) During the intervening time, she referred to dating a doctor (Milligan). (TV: Reset) She is later said to be on honeymoon. (TV: Children of Earth: Day One) At some point, she eventually marries Mickey Smith. (TV: The End of Time)
 * When the Sontarans realise they had an intruder, the Doctor replies, "How'd he get in, then? In-tru-der-window?" Henry van Statten said the same thing regarding the Ninth Doctor. (TV: Dalek)
 * Staal remarks about the Sontarans being forbidden to be involved in the Last Great Time War. It was most possibly because of the Sontaran Invasion of Gallifrey under Commander Stor of the Special Space Service. (TV: The Invasion of Time) Some Sontarans tried to get involved in the Time War. (AUDIO: The Sontaran Ordeal, The Eternity Cage)

Home video releases

 * This story was released in the Series 4 DVD box set in November 2008 along with the rest of the series.
 * It was released as Series 4 Volume 2 in a vanilla edition with The Poison Sky, The Doctor's Daughter and The Unicorn and the Wasp.