The Christmas Invasion (TV story)

The Christmas Invasion was the first episode of Doctor Who to premiere on Christmas Day since "The Feast of Steven", the seventh part of the 1966 epic twelve-part serial The Daleks' Master Plan. However, unlike that episode, The Christmas Invasion was specially commissioned by BBC One to be transmitted outside the programme's normal broadcasting season. It was thus the first in the modern tradition of the "Christmas special", and its sixty-minute running time made it then the longest episode yet produced by BBC Wales.

Narratively, it continued the story of Harriet Jones, started the Torchwood story arc and involved UNIT in its first major appearance in the new series.

More importantly, it was the first full story to feature the Tenth Doctor, played by David Tennant. Its initial, pre-titles sequence on the Powell Estate was later used to "bookend" the closing scenes of The End of Time. Its setting was the first and last place on Earth the Tenth Doctor saw.

Synopsis
It's Christmas Eve and high above London, the alien Sycorax are holding the Earth for ransom. The Tenth Doctor must recover from his regeneration in time to save the human race from slavery.

Plot
It is Christmas Eve on Earth. As Jackie prepares presents and Mickey works in the garage, both of them hear the distinctive sound of the TARDIS' engines. Rushing out into the street of the Powell Estate, they see the TARDIS blink into existence above them, ricochet off a few buildings and a post van, then come to a crashing halt. A strange man stumbles out of the police box doors, greets them by name and wishes them a merry Christmas before collapsing. Rose follows and, in response to Jackie and Mickey's questions, identifies the stranger as the Doctor.

They bring the Doctor to Jackie's flat and dress him in pyjamas belonging to Howard, Jackie's current beau, who has the habit of keeping pieces of fruit in his pocket for snacks. While Rose discusses the Doctor's change of appearance and the fact he has two hearts with Jackie, they do not see a wisp of vortex energy emerging from the Doctor's mouth, which then floats into space. On television, Prime Minister Harriet Jones and project director Daniel Llewellyn give a press conference about the Guinevere One space probe, which is about to land on Mars. In space, however, the probe is swallowed up by an island-like spaceship.

That evening, Rose and Mickey go Christmas shopping, but are attacked by a group of masked Santas armed with lethal musical instruments. Managing to escape when the tuba mortar brings a giant Christmas tree down on the Santas, Rose realises that the Santas must be after the Doctor. She and Mickey rush home. When they reach the flat, Rose notices an unfamiliar Christmas tree in the sitting room, which Jackie says was delivered to the door. As they realise that none of them purchased the tree, it comes to life, whirling around with razor-sharp branches.

The three retreat to the bedroom, the "Christmas tree" in hot pursuit. Rose places the sonic screwdriver in the still-comatose Doctor's hand and asks him to help her. Reacting instinctively, the Doctor rises as the tree bursts through the door and disintegrates the tree with the screwdriver. He then strides outside the flat to see who was remotely controlling the tree. From ground level, the Santas stare up at the Doctor, but transmat away when the Doctor points the sonic screwdriver at them. The Doctor calls them "pilot fish" and collapses in pain, saying that Rose woke him up too soon: he is still regenerating. The energy leaking from him has attracted attention, and if the “pilot fish” could trace it, then something bigger is coming. He then loses consciousness again. The first signal from Guinevere One arrives: a distinctly alien face, which is soon broadcast all over the world. Llewellyn is escorted by Major Blake to the Tower of London, which houses a facility run by the United Nations Intelligence Taskforce. There, he meets the Prime Minister and her aide, Alex, who tells him that the cover story is that a student in a mask hacked into the television signal. Llewellyn is shaken to realise that extraterrestrial life does exist, and that both the British government and the United Nations are aware of this. A technician, Sally Jacobs, explains that the signal did not come from Mars but 5000 miles above the planet's surface, which means that there is a ship, and it is moving rapidly towards Earth.

As Rose and Mickey use his laptop to monitor UNIT's readings, the aliens send another signal. The aliens speak in their own language, but Rose does not understand it. Normally, the TARDIS would translate it for her, but it seems that with the Doctor unconscious, that function is not working.

At UNIT, Blake orders the use of translation software. With no sign of the Doctor, Jones asks Blake about "Torchwood". She knows that she is not supposed to know about them – not even the United Nations knows — but she wants them to be ready.

The software rather imprecisely translates the message. The aliens are the Sycorax, and they are claiming the planet as their own, demanding surrender or "they" will die. Their word for "human" also appears to be similar to that of "cattle", temporarily baffling UNIT. Jones declines to surrender, warning the Sycorax that the planet is armed. As dawn rises over London, the Sycorax respond. With a wave of the leader's hand, blue energy sweeps over a third of the world's population, mesmerising them. The mind-controlled people, Sally Jacobs amongst them, climb to the highest spots they can find (primarily the roofs of buildings), and stand at the edge, poised to jump.

Checking the UNIT staff's medical records, Llewellyn discovers that all the affected people have A+ blood. The Sycorax found the sample of A+ blood that was sent with other materials on Guinevere One to identify the human race in case of alien contact, and are somehow using that as a control mechanism. Desperate now, Jones gives an emergency broadcast on television, pleading for the Doctor's help if he is out there. She also informs the public that the Queen's Christmas speech has been cancelled and finds that the Royal Family are "on the roof". Just then, the shockwave of the Sycorax ship entering the atmosphere shatters windows all over the city; the gigantic craft takes position above the centre of London as the frightened population watch. Rose, driven to despair by the Doctor's comatose state and not knowing what else to do, asks Mickey and Jackie to help move the Doctor to the safety of the TARDIS. Jackie gathers food and other supplies, including a thermos flask of tea.

The Sycorax transmat Jones, Alex, Blake and Llewellyn up to their ship. The Sycorax leader removes his helmet, revealing a skinless face surrounded by a mantle of bone. His hand hovering over a large glowing button, he demands immediate surrender, or he will order the controlled humans to jump. Llewellyn tries to reason with the Sycorax, but is reduced to a pile of bones by the leader's energy whip, as is Blake when he protests. Half of the world will be sold into slavery or a third will die; it is Jones's choice.

As Rose and Mickey move the Doctor into the console room, Jackie goes back to get more supplies. Rose, having apparently given up, broods by the console as Mickey tries to use the TARDIS scanner to tune into what is happening, but the time machine's advanced technology is detected by the Sycorax. Outside, Jackie watches helplessly as the TARDIS is transmatted up. Not realising that they are aboard the Sycorax ship, Rose steps out of the TARDIS, and screams when she sees the aliens. Mickey rushes out after her, dropping the flask of tea, which spills and starts dripping through the grilles at the base of the console next to the Doctor's unconscious form. The Doctor breathes in the fumes created as the tea sparks against various components.

Rose tries to bluff the Sycorax by quoting various things and races she has encountered on her travels, commanding them to leave, but is answered with laughter. The Sycorax leader taunts her attempts to pass off second-hand knowledge as authority, but as he gloats, his alien words start turning into English. Rose realises that the TARDIS translation is working again, and as the Doctor must be conscious for it to be active, that can mean only one thing: the Doctor is awake. On cue, the doors of the police box open and the Doctor stands there, smiling as he says, "Did you miss me?"

Easily deactivating the Sycorax leader's energy whip and breaking his staff, the Doctor bluntly tells the alien to wait while he gets more important things out of the way, namely, getting reacquainted with his friends. Disappointed at not being "ginger" (red haired), and somewhat annoyed at Rose's speed in giving up on him, he tells them that all he needed was a "good cup of tea; a superheated infusion of free radicals and tannin. Just the thing for healing the synapses." As the Sycorax leader demands to know who he is, the Doctor blithely strides across the ship's floor, nattering on cheerfully and still working out what his personality is like in this new incarnation. He walks up to the glowing button, discovers that it is powered by A-positive human blood, and quickly deduces that the Sycorax are using blood control — they're controlling all the humans with A-positive blood. The Doctor tells the leader that in his unstable state, when he sees a large glowing button he just cannot help himself — and to everyone's shock, he pushes it. However, instead of sending the possessed crowds on Earth to their deaths, it simply releases them from the Sycorax control. The Doctor explains that blood control is like hypnosis: you cannot hypnotise a person and convince them to kill themselves as the survival instinct is too strong. The Sycorax were bluffing, and the Doctor merely called them on it. The leader says that they can still conquer Earth with an armada, but the Doctor demands that the humans be left alone (quoting part of "The Circle of Life" from The Lion King in the process), ultimately challenging the leader to single combat for the planet. The swordfight goes from inside the ship to its exterior. In the midst of it, the leader cuts the Doctor's hand off. However, the Doctor is still in the first 15 hours of his regeneration cycle, and regrows his hand. He gains an advantage over the Sycorax leader and triumphs. Holding the leader at sword point at the ship's edge, the Doctor extracts an oath from the leader to leave the planet and never return, in return for the Doctor sparing his life. As the Doctor walks back, celebrating his victory with Rose, the leader tries a final attack whilst the Doctor's back is turned. The Doctor calmly bounces a satsuma he finds in Howard's dressing gown off a control button, opening a section of the ship's wing beneath the leader, sending the alien plunging to his death. The new Doctor is not a man willing to grant second chances.

The Doctor sends the other Sycorax on their way with a reminder that the planet Earth is defended. They are transmatted back to London, and Jones asks if there are more aliens out there. The Doctor notes that there are thousands; the human race is being noticed more and more. As Jones ponders this, visibly troubled, Alex receives a telephone call and quietly informs Jones that Torchwood is ready. Jones seems reluctant, but nevertheless gives the order to fire. Five green beams converge as one over London, and the resulting energy burst destroys the Sycorax ship as it heads into space.

The Doctor glares at Jones, furious, but she tries to justify the use of the weapon (engineered from a crashed spaceship ten years previously) as defending the planet, especially since the Doctor cannot be there all the time. The Doctor bitterly says he should have warned the Sycorax to run, as the real monsters, the humans, are coming. When Jones asks if she should consider the Doctor another alien enemy, the Doctor warns her that he can bring down her government with just six words. He whispers them in Alex's ear: "Don't you think she looks tired?" Jackie, Mickey and Rose serve Christmas dinner in the flat. The Doctor looks through the TARDIS wardrobe, finally settling on a brown pinstripe suit and a long brown coat. He joins the others for dinner, and they watch Harriet Jones on the television, fending off rumours about her ill-health and a pending vote of no confidence in the House of Commons. Outside, what looks like snow is falling over London, accompanied by shooting stars, but the Doctor points out that it is, in fact, ash — the remains of the Sycorax spaceship. It is a new start for Earth, however; with so many people seeing the Sycorax ship, there is no covering up the existence of aliens this time.

But there are new worlds to see and explore. With a now-trusting Rose by his side and eager to continue their travels, the Doctor looks up into the sky to choose a star for their next destination, assuring her that it will be, in the words of his previous incarnation, "fantastic".

Cast

 * The Doctor - David Tennant
 * Rose Tyler - Billie Piper
 * Jackie Tyler - Camille Coduri
 * Mickey Smith - Noel Clarke
 * Harriet Jones - Penelope Wilton
 * Danny Llewellyn - Daniel Evans
 * Alex - Adam Garcia
 * Sycorax Leader - Sean Gilder
 * Major Blake - Chu Omambala
 * Sally - Anita Briem
 * Alan - Marvyn Williams
 * Sandra - Sian McDowell
 * Jason - Paul Anderson
 * Mum - Cathy Murphy
 * Policeman - Sean Carlsen
 * Newsreader 1 - Jason Mohammad
 * Newsreader 2 - Sagar Arya
 * Newsreader 3 - Lachele Carl

Uncredited cast

 * Geoffrey Baxter - Ian Hilditch

The Doctor

 * When Rose asks him to help her, the unconscious Doctor snaps awake to protect her.
 * The Doctor is healed by the exposure to free radicals in vapour form created by steeping tea.
 * The leader of the Sycorax slices off the Doctor's hand. However, the Doctor regenerates the hand, as he is still in the first 15 hours of his regeneration cycle.
 * The tenth incarnation of the Doctor assumes he has become rude, and expresses dismay that his is not a ginger. He also retires his catchphrase "Fantastic", used during his previous life, but fondly uses it one last time to remind Rose he's still the same man on the inside.
 * The Doctor says he gives no second chances, as he is "that sort of a man".
 * The Doctor is still skilled at sword-fighting and also has remarkable throwing skills regarding his aim with round objects, such as a satsuma.

Organisations

 * Mickey works at Clancy's.

Galactic law

 * Rose mentions the Shadow Proclamation.

Biology

 * The Doctor recognises the human blood and its blood type by taste.

Cultural references from the real world

 * The song "Rudolph the Red-Nosed Reindeer" is playing.
 * The Doctor mentions the Disney animated feature/stage musical "The Lion King" after quoting lines from the song "Circle of Life".
 * A signal intrusion incident that occurred in Chicago during an airing of the Fourth Doctor serial Horror of Fang Rock is referenced by Harriet Jones' aide Alex when he says the official story of the Sycorax appearing via the probe is because of "a mask, some sort of prosthetic. Students hijacking the signal."

Foods and beverages

 * The chemical components in tea can complete the healing of brain synapses and neurons recovering from the regeneration process.
 * The Doctor finds an apple in the pocket of Howard's bathrobe.
 * The Doctor uses a satsuma (also found in the pocket of Howard's bathrobe) to defeat the Sycorax Leader.

Planets

 * Guinevere One was en route to Mars when intercepted by the Sycorax ship.

Species

 * Blake says, "Martians look completely different", a possible reference to Ice Warriors.
 * Daleks, Slitheen, and the Gelth are all mentioned by Rose.

Time Lords

 * While suffering from regeneration sickness, the Doctor says that he is "having a neuron implosion." Later, in reference to the cure for this malady, he mentions synapses.
 * Because he is within the first fifteen hours of his regeneration cycle, the Doctor has enough residual energy to regrow his severed hand.

Torchwood

 * This is the first time the Torchwood Institute is acknowledged to exist as a physical entity; it was first mentioned during The Weakest Link game on the Game Station. It is so secret even the United Nations does not know of its existence, and the Prime Minister isn't supposed to know, although Harriet Jones somehow does.

United Nations Intelligence Taskforce

 * UNIT has a headquarters in the Tower of London.
 * According to Harriet Jones, a "Code 9" refers to the Doctor's involvement.

Technology

 * The Hubble Array is following the course of the Sycorax ship.

Story notes

 * This is the first Doctor Who episode clearly labelled as a Christmas special. However, the seventh episode of The Daleks' Master Plan, titled "The Feast of Steven", was also written as a Christmas episode and was first broadcast on 25 December 1965.
 * This is the first episode of Doctor Who to air on a Sunday.
 * David Tennant is credited as 'The Doctor', as opposed to Christopher Eccleston who was credited as 'Doctor Who'. The change in the credit was done at Tennant's request.
 * The 'middle eight' section of the theme tune is restored in this episode and is heard for the first time in the revived series.
 * Before this episode was broadcast, a fictional tie-in website for the Guinevere One project was created and launched by the BBC. The site includes an introduction by Harriet Jones and an interview with the project director, Professor Daniel Llewellyn. The site claims that the probe was developed by the British Rocket Group. The organisation's logo partially appears in this episode, in the televised press conference with Professor Llewellyn. The name of the organisation was first mentioned in Remembrance of the Daleks and is a reference to the British Experimental Rocket Group from the Quatermass science fiction serials of the 1950s. David Tennant had previously starred in the 2005 BBC live remake of as Doctor Gordon Briscoe, and Quatermass' first line to Briscoe was changed by actor Jason Flemyng during the broadcast from "Good to have you back, Gordon" to "Good to have you back, Doctor" — Tennant's casting as the Doctor was announced two weeks after Quatermass went to air, and his castmates would have been aware of the speculation during rehearsals.
 * Immediately following this episode Attack of the Graske (an interactive mini-episode starring David Tennant) was made available on the BBC Red Button.
 * Just before the opening credits sequence, Jackie says the line, "Doctor? Doctor who?", continuing a long-running in-joke. Unlike many instances of the joke, however, the line primarily plays as a genuine question, since she has never before met the Tenth Doctor and knows nothing of regeneration.
 * The Tenth Doctor speaks with an accent similar to Rose's but unlike the Ninth Doctor's Northern one. In a radio interview broadcast on 23 December 2005, Tennant explained that a line of dialogue had been scripted for this episode which explained that the newly regenerated Doctor had imprinted on Rose's accent, "like a chick hatching from an egg", but the line was deleted from the final episode.
 * The opening shot of this episode, in which the Earth and its moon appear, is reused footage from the opening shot of TV: Rose.
 * According to press reports released before this episode was broadcast, producer Russell T Davies stated that he believed Christmas specials should include traditional Christmas items such as sleigh bells, snow, reindeer, and Santa.
 * The song being played by the Santa Claus band which attacks Rose and Mickey is "God Rest Ye Merry, Gentlemen". This song is traditionally associated with the novel A Christmas Carol, written by Charles Dickens, whom the Doctor and Rose met in The Unquiet Dead. The song shares its melody with the "Venusian Lullaby" the Third Doctor sang in TV: The Dæmons and The Curse of Peladon. The carol can be heard again in TV: The Next Doctor.
 * Another song featured in this episode is "Song for Ten", an original composition by Murray Gold sung by Tim Phillips. The next two Christmas specials included an original song on the soundtrack. The Phillips version of the song was very brief, and when the time came to compile a soundtrack album, new lyrics were written for the song (reflecting the events of Doomsday) and it was recorded by Neil Hannon. "Song for Ten" is the first original song commissioned for Doctor Who since the untitled rap song for the Ringmaster heard during the 1988 story TV: The Greatest Show in the Galaxy.
 * David Walliams and Bill Nighy were considered for the role of the Tenth Doctor.
 * The original choice for the role of the Tenth Doctor was an unknown (and unnamed by the BBC) English actor who spoke in cockney accent, he didn't get the role because he moved with his family to Australia.
 * One of the outfits considered by the Doctor in the wardrobe is the costume worn by David Tennant in his previous role as Casanova in.
 * When the Doctor is leaving the wardrobe near the end of the episode, a long coat and long yellow and red scarf (possibly Tom Baker's famous scarf) can be seen hanging from the very hat rack that appeared in the TARDIS in the Tom Baker years.
 * The laser weapon used to destroy the Sycorax spaceship was similar to the one used by the Death Star in Star Wars Episode IV: A New Hope and Star Wars Episode VI: Return of the Jedi.
 * This marks the first time in the new series that any room in the TARDIS other than the console room is seen on-screen.
 * In a scene filmed for the episode, but deleted before broadcast (though included on the DVD), the Doctor attempts to utter his predecessor's catchphrase, "Fantastic!" but due to his "new teeth" (ref. his comments at the end of The Parting of the Ways) finds it initially impossible to do so. This scene was intended to set up the final scene of the episode in which the new Doctor finally utters the word, "Fantastic!"
 * The Christmas Invasion can be viewed in its entirety on the BBC America website.

Ratings

 * 9.84 million

Myths

 * Some early reports suggested that the enemy would be the Cybermen. Tabloid newspaper reported that Shaun Dingwall would return as Rose's father, Pete Tyler, and that this episode would be set on an alternate Earth. However, all of these claims were proven to be incorrect when the episode was broadcast, and appear to refer to Rise of the Cybermen/The Age of Steel. Coincidentally, the Cybermen were later announced as the enemies in the later 2008 Christmas special.
 * After the announcement that Christopher Eccleston would leave after the first series, there were erroneous reports that the regeneration was to take place during the Christmas special, not during The Parting of the Ways. (This may have been an intentional red herring in order to maintain at least some element of surprise for the series finale given that the BBC had accidentally blown Russell T Davies' plan for a surprise regeneration.)

Influences

 * Besides being the name of the invading aliens, Sycorax is also the name of the witch in Shakespeare's play, . In the later series 3 episode, The Shakespeare Code, the Doctor accidentally gives Shakespeare the name "Sycorax" when he sees an animal skull which reminds him of one of the aliens.
 * The Sycorax, with their curse-like blood control technology and bone-motif costumes, are slightly similar to Faction Paradox, a time-travelling voodoo cult created by Lawrence Miles that were recurring villains in the BBC Eighth Doctor Adventures novels.
 * Harriet Jones's decision to destroy the Sycorax spaceship, despite the fact it was leaving Earth, echoes Margaret Thatcher's decision to sink the Argentinian ship Belgrano in the 1982 Falklands War, even though evidence suggests it was actually leaving British waters.
 * Guinevere One, the name of the probe that Earth sends to Mars, references the myths of King Arthur. In those stories, Guinevere was Arthur's Queen Consort. Her name is an old French form of the Welsh name Gwenhwyfar, which can be translated as "white shadow". Her adulterous affair with Arthur's chief knight, Lancelot, and betrayal of her husband lead to the downfall of their kingdom. When the Seventh Doctor was recognised as Merlin, Arthur's advisor, he assumed that he would later find himself in this role. (TV: Battlefield)
 * The Doctor's sword duel with the Sycorax leader, particularly when his hand is cut off, suggests the lightsaber duel between and  in .
 * The Doctor comments that the bathrobe and pyjamas Jackie has given him to wear are "very Arthur Dent." Arthur Dent is a character in The Hitchhiker's Guide to the Galaxy, created by former Doctor Who script editor Douglas Adams. Also, Jackie's line "Anything else he's got two of?" is a reference to the same line spoken by Arthur Dent in the 2005 movie adaptation of The Hitchhiker's Guide to the Galaxy.
 * When speaking to the Sycorax of the great potential humans possess, the Doctor realises he is quoting from The Lion King.

Filming locations

 * Tredegar House, Newport
 * Brandon Estate, Kennington, London (The Powell Estate)
 * Palace of Westminster, Westminster, London
 * Tower of London, London
 * Landmark Place, Churchill Way, Cardiff
 * Hayes Island, Cardiff
 * Clearwell Caves – Ancient Iron Mines, Coleford, Gloucestershire
 * Barry Docks, Barry Island, Cardiff
 * NCP Tredegar Street (also known as St Davids 2), Cardiff, Wales
 * Wallis House, Great West Road, Brentford
 * Trafalgar Square, London
 * 30 St Mary Axe (also known as "The Gherkin"), London
 * Baltic House, Mount Stuart Square, Cardiff
 * Wharton Street, Cardiff
 * Broadstairs Road, Leckwith, Cardiff
 * Brian Cox Motor Engineering, Bromley Road, Ellwood (Clancy's garage where Mickey is working when the TARDIS arrives in the teaser sequence)
 * Millennium Stadium, Cardiff
 * Unit Q2, Imperial Park, Imperial Way, Newport
 * HTV Wales Studios, Culvershouse Cross, Cardiff
 * BBC Kendal Avenue, Kendal Avenue, Acton
 * BBC Broadcasting House (C2 Studio), Llantrisant Road, Llandaff, Cardiff

Production errors

 * After the Doctor changes his clothes and looks in the mirror, the TARDIS doors are behind it leaving a possible indication that they used the same set as the TARDIS console room.
 * Although there has been a shot of the Big Ben clock tower showing it to be whole since the damage caused by the crash during the Slitheen takeover at Downing Street, it is still under repair in this story. (TV: Aliens of London, World War Three)
 * After Blake is killed, it is the same charred remains as those used for Danny Llewellyn.
 * When Mickey calls Rose to check out the military broadcast of the Sycorax ship, you can see for a split-second the video player playing the broadcast, revealing that it was not green-screened, but a video playing on the computer.
 * Although it has been less than an hour since the Doctor regenerated, as Rose passes the door of the TARDIS after the Doctor collapses, her hair is visibly longer. (TV: The Parting of the Ways) This exposes the several month break between the wrapping up of Series 1 production and the start of Series 2 production, during which time Billie Piper allowed her hair to grow longer. A similar occurrence happened with actor Elisabeth Sladen in TV: Invasion of the Dinosaurs, though for the opposite reason of having her hair cut shorter for a fashion photo shoot. Coincidentally, both stories with a hair-related discontinuity feature the word "Invasion" in their title.
 * Although the proportion of A+ people in the UK (35%) is close to one-third, the world-wide proportion of A+ people is 28.3%, which is close to two-sevenths. Interestingly enough, at one point in the dialogue it's mentioned that there are two billion people on rooftops and that this number is one-third of the world's population -- but that's actually closer to two-sevenths.
 * When proclaiming his new form to be "lucky", the Doctor is facing towards the Sycorax ship, but when mentioning being in the first 15 hours of his regenerative cycle, he is facing away from the Sycorax ship. He is facing the ship again when his hand regrows.

Continuity

 * The Doctor's speech to PM Harriet Jones about how "Earth is drawing attention to itself" recalls a similar statement made by the Brigadier in TV: Spearhead from Space. Jones's destruction of the Sycorax ship and the Doctor's angry reaction are similar to the conclusion of TV: Doctor Who and the Silurians, in which the Brigadier uses explosives to seal off the Silurian hibernation chambers even as the Doctor departs to begin peace negotiations.
 * United Kingdom had manned missions to Mars previously featured in TV: The Ambassadors of Death and PROSE: The Dying Days.
 * The Ice Warriors are actual Martians. (TV: The Ice Warriors)
 * The Robotic Santas reappear in TV: The Runaway Bride.
 * The Doctor regrows his severed hand, establishing a notable new twist to the mechanics of regeneration and, in part, suggesting a rationale for Romana II's ability to change her appearance several times during regeneration in TV: Destiny of the Daleks. The Doctor's severed hand, last seen falling to earth in this episode, is later retrieved by Jack Harkness. (TV: Everything Changes, TV: Utopia) Much later, it would return to the Doctor. (TV: Last of the Time Lords) After the Doctor used it as a receptacle for regeneration energy, something compelled Donna to touch the hand, enabling it to grow into the Meta-Crisis Tenth Doctor (and also turning Donna into the Doctor-Donna). The real Tenth Doctor subsequently took the Meta-Crisis Tenth Doctor to the parallel universe, leaving him to live out his life there with Rose Tyler. (TV: Journey's End)
 * This is actually the second time the Doctor has lost a hand in combat, as the First Doctor sported a bioengineered replacement hand for most of his adventures. (PROSE: A Big Hand for the Doctor)
 * In the scene inside the TARDIS' wardrobe room near the end of the episode, various articles of clothing worn in earlier episodes are visible, including one of Steven Taylor's shirts (worn in TV: The Celestial Toymaker), what may be the Sixth Doctor's waistcoat from The Two Doctors, the Second Doctor's trousers, and the Fifth Doctor's panama hat, as well as what appears to be the Fourth Doctor-like costume that the Seventh Doctor wore in TV: Time and the Rani.
 * The ramifications of the destruction of the Sycorax vessel, including the impact on a group of female (and presumably related) Sycorax, is explored in COMIC: The Widow's Curse.
 * This is one of the few instances in which the TARDIS isn't stationary as it materialises, due to the Doctor speeding it up to breakneck speeds in the time vortex; (TV: Children in Need Special) in most other episodes, the TARDIS stays in one spot as it de- and rematerialises. A similar "materialisation in motion" occurs on two occasions during the next Christmas special, TV: The Runaway Bride.
 * Jones' statement regarding the Doctor not always being available to help the planet touches on a theme that is revisited in TV: Children of Earth: Day Five.
 * When the Doctor collapses the first time and Mickey questions who he is, the Doctor's position mirrors the position of the Fifth Doctor in TV: The Caves of Androzani.
 * The Third Doctor was revived from a trance by coffee in TV: Planet of the Spiders.
 * The Doctor is able to identify blood type through taste. In TV: Destiny of the Daleks, Romana demonstrated a similar sensory precision in identifying a mineral's composition through taste.
 * When the Doctor regenerates into the Eleventh Doctor in TV: The End of Time, he complains again when he notices that he's "still not ginger", as he does in this story.
 * A panoramic shot of London reveals construction girders around the Clock Tower of Big Ben, indicating that repair work is still being done following its collision with a spaceship in TV: Aliens of London.
 * The Sycorax would later appear in TV: The Pandorica Opens as part of the Alliance.
 * Harriet Jones introduces herself to everyone as "Harriet Jones, Prime Minister" to which whomever she is talking to replies, "Yes, I know who you are", a reference to TV: Aliens of London and World War Three in which she introduces herself to everyone as "Harriet Jones, MP for Flydale North". A similar line would also be used in TV: The Stolen Earth.
 * The Doctor has previously engaged in sword fights. (TV: The Androids of Tara, et al.)
 * By seeding the downfall of Harriet Jones as Prime Minister, the Tenth Doctor unwittingly triggered a cascade of events that end with his demise:
 * The Doctor and future companion Martha Jones arrived at Malcassairo in the year 100,000,000,000,000 by accident. They also found that Jack Harkness hitched onto the TARDIS exterior and was dragged through the Time Vortex, trying to reunite with the Doctor after being left behind at the Game Station when he was made immortal by the Bad Wolf. (TV: The Parting of the Ways, Utopia)
 * The group met Professor Yana, who Martha noticed had a fob watch. After her experience with the chameleon arch, she thought Yana was a benevolent Time Lord that became human, but Yana's true identity belonged to the Master, who had fled the Time War. After regenerating, he stole the Doctor's TARDIS and fled the planet, but the Doctor tampered with his TARDIS to trap him on Earth. (TV: Human Nature, Utopia)
 * Jones' removal from power allowed The Master to replace her under the identity of Harold Saxon, though he was later removed from power and shot by Last of the Time Lords (TV story) for tormenting her. He chose to die rather than regenerate into a life of captivity. (TV: The Sound of Drums, Last of the Time Lords)
 * The Disciples of Saxon, the Master's followers, resurrected him, but Lucy again tried to kill him and managed to damage his new body at the price of her own life. The crippled Master was soon recruited by Joshua Naismith to work on the Immortality Gate for his intellect. The Master sabotaged the Gate and used it to make the Master Race, then retrieved Gallifrey and its denizens from the Time War with the help of a White-Point Star. (TV: The End of Time)
 * During the Master's confrontation with Lord President Rassilon, Wilfred Mott rescued a trapped worker in the Gate's radiation booth and took his place. The booth's anti-leak security measures only allowed one person to exit while another person had to be locked inside. (TV: The End of Time)
 * The Doctor cut the link holding Gallifrey out of the Time War, which sent the Time Lords back into the battlefield, the Master joining them. However, he left the Immortality Gate's nuclear bolt running. It went into meltdown and the excess radiation would have dumped into the containment booth with Wilf still inside. Trying to operate the booth in any way would also have the same result. In order to rescue Wilf, the Doctor had to enter the booth and expose himself to the radiation, leading to his regeneration. (TV: The End of Time)

Home video releases

 * A behind-the-scenes preview of this episode was released with the series 1 DVD box set.
 * As with every other episode of season 2, The Christmas Invasion was released on VHS and DVD by Columbia Tristar Home Entertainment.
 * This story was released on a vanilla DVD with New Earth.
 * It was also released as part of the Series 2 DVD Box set by Columbia Tristar Home Entertainment in July 2006.
 * This story was also released with Issue 7 of the Doctor Who DVD Files.