It was notable for featuring the first cliffhanger in the BBC Wales run, by virtue of starting the first two-part story. Keeping cliffhangers in the show allowed Davies to expand the plot and restore a well-remembered part of the original series.
Narratively, the episode had several introductions. It introduced the Slitheen, who would return to Doctor Who and The Sarah Jane Adventures. It also marked the first appearance of Toshiko Sato, who would later feature as a regular character on Torchwood. In addition, this episode introduced another recurring character, British politician Harriet Jones. It also introduced UNIT to the revived series.
One of its legacies — sometimes forgotten by writers — was offered in its pre-title sequence. As the Ninth Doctor mistakenly brought Rose Tyler one year into the viewer's future, the "present day" of Doctor Who and its two BBC Wales spinoffs were brought, until Planet of the Dead, to one year later than the year of initial broadcast.
It was also the final episode of Doctor Who to have any portion recorded at the BBC Television Centre.
Rose returns home to discover that she has been missing for a whole year, although for her, it's been a couple of days. However, before she can explain her absence, a spaceship crashes into Big Ben, causing a worldwide crisis. Worse still, the Prime Minister has mysteriously disappeared... The Doctor's investigation puts him in the spotlight with the British government, as his long history of defending Earth finally catches up with him. But there are sinister goings on at 10 Downing Street, and politician Harriet Jones' quest to get some answers brings her into a brave new world... of aliens. Meanwhile, Rose finds trouble closer to home, as her past mistakes threaten to tear her family apart.
Rose and the Ninth Doctor have returned to the Powell Estate. Rose, thinking she has only been gone twelve hours, heads off to see her mum. Meanwhile, the Doctor notices a poster depicting a missing girl. Jackie is shocked to see Rose, who is lying that a friend of hers needed to talk to her over night. As Rose returns her mother's hug, she notices several posters in the living room that say she is missing. A breathless Doctor enters the room and informs Rose that she's been gone twelve months, not twelve hours.
Later, a policeman questions the Doctor; Jackie called the police as she did not believe Rose's claims to have been travelling as her passport is still at home. The Doctor explains that he employed Rose as his travelling companion; they simply lost track of time. The policeman inquires as to if this term refers to anything sexual in nature, to which both the Doctor and Rose retort "no." Jackie questions if the Doctor is really a doctor, or simply made up stories to lure in Rose for some kind of vile purpose. The Doctor tells her that he is one, to which she says "Prove it, stitch this mate" and slaps him.
Later, Rose and the Doctor chat outside on the roof of the Powell Estate. The Doctor is surprised Jackie slapped him, remarking his companion's mothers have never slapped him throughout his 900 years of travelling through time and space. Rose notes that's "one hell of an age gap" between them. She then laments that she's seen so much in her travels with him, but she can't tell anyone. Both are shocked to see an alien spaceship pass overhead and clip Big Ben before crashing into the river.
Rose and the Doctor try to visit the scene but are prevented by traffic; Rose suggests the TARDIS, but the Doctor tells her that is a bad idea as everyone in the world is now watching the skies for more aliens. They head for home to watch the news on television. Several people are visiting, while the Doctor tries to watch the news. He sees General Asquith entering the hospital where the alien has been taken. The news also reports that the Prime Minister is still missing.
At the hospital mortuary, Dr Sato performs an autopsy on the alien. Meanwhile, several important figures gather at 10 Downing Street, including Joseph Green. He is informed by Indra Ganesh, the junior secretary, that he is acting Prime Minister for the crisis. As Ganesh attempts to lead Green away, he is accosted by Harriet Jones, MP for Flydale North, who wishes to meet with the Prime Minister but is brushed off. Green meets with Margaret Blaine and is given the emergency protocols.
The Doctor decides to leave the party, giving Rose a TARDIS key; he tells her that the crash was genuine, as it gave all the signs of engine failure making that day is the day the human race learns that they are not alone in the universe, and can start becoming part of the greater world beyond Earth. He promises he won't interfere, but is just going for a wander as the atmosphere inside the flat is 'too human'.
Despite his assurances, the Doctor decides to investigate when Mickey notices him. Giving chase, he reaches the TARDIS just as it disappears. The Doctor heads to the hospital where the alien is being held. He accidentally walks into a room of soldiers, but when they hear screaming, the Doctor calls out a battle formation and they all run out of the room and the soldiers listen to the Doctor's orders. He finds Dr Sato, who says that the "dead" alien is alive and has run off. Giving chase, they find that the alien has the appearance of a pig, albeit one running on its hind legs and wearing a suit. It runs away in obvious fear, and the Doctor tries to catch it. Despite the Doctor's command, one of the soldiers shoots the alien dead. Saddened, the Doctor tells the soldiers it was just scared and trying to run away.
Back in the Cabinet Room, General Asquith meets with Green and Blaine, who are acting strangely. He attempts to relieve Green of command, but Green, Blaine and another man, Oliver Charles, unzip their foreheads. Asquith screams. Unknown to them, Harriet Jones has sneaked inside the room and watches them from her hiding place, as they kill him.
After examining the body, the Doctor informs Dr Sato that the pig is (or rather was) an ordinary pig from Earth. It had its brain rewired and was stuck in the ship which was sent to dive bomb into the Thames. Although its obvious the crash was faked, the technology involved is indeed not from Earth. Sato questions why aliens would fake an alien encounter, only to find the Doctor has gone. Meanwhile, Mickey arrives at Rose's flat, revealing that several people, including Jackie, had suspected he murdered Rose. Sneering at Jackie, Mickey tells Rose that Jackie called the police three times because she thought he killed Rose though there was no any evidence of such, but the accusations ruined his reputation.
Outside, as Mickey is gloating that the Doctor has left Rose, the TARDIS materialises. The trio enter but the interior causes Jackie to flee back to her flat and call a number to report the Doctor. When she mentions the word 'TARDIS', an alarm is triggered After a brief spat with Mickey about his name really being "Ricky", the Doctor tells her that he has deduced that the spaceship landing was faked; it was launched from Earth, and whoever did it has been here for a while. Mickey points out that all the crash has done is put the world on red alert, which is unusual if the aliens are planning to invade. He notes that UNIT has been called in to deal with the crisis.
The trio of Green, Blaine and Asquith complain about compression and gas exchange. They receive a code nine — the Doctor, an expert in extra-terrestrial affairs, has been located. On the TARDIS, the Doctor finds that experts and specialists on alien encounters are being brought in and realises that these people belong to UNIT. He fills Rose in on the organisation and Mickey confirms that the Doctor has worked for them before, having spent the past year researching the Doctor's past history on Earth. Rose suggests going to UNIT for help, but the Doctor refuses as the world doesn't need another alien (and also points out he's 'changed a lot' since his last meeting with them).
The Doctor decides they need to check out the crashed spaceship up close now the roads are clearing. However, when the Doctor, Rose and Mickey step out of the TARDIS, they are surrounded by police and helicopters. Mickey runs away, and the Doctor and Rose are taken in; however, the Doctor tells Rose that they're not being arrested but instead escorted to the heart of the discussion. He's the ultimate expert on aliens, according to UNIT files. They are taken to Downing Street, where the Doctor is brought in to meet with the other experts. Rose does not have proper clearance, so Harriet offers to stay with her. Meanwhile, a policeman visits Jackie for information.
The experts are gathered with Green and Asquith in a small room. Harriet takes Rose to the Cabinet Room and tells what she saw — Green and Blaine are aliens in skin suits who killed Asquith. Suddenly the two find the body of the Prime Minister hidden in the cupboard.
The Doctor interrupts Asquith telling everyone that three days earlier, some radiation was detected in the North Sea and was due to be investigated but this was forgotten after the crash. The Doctor wonders aloud why the spaceship crash was faked, since all its managed to do is panic the population and get the Earth's alien experts in one place. The Doctor suddenly realises that this is what the aliens want; anyone who has the expertise and knowledge to fight them gathered together. He now understands the crash wasn't a diversion but a trap. The policeman visiting Jackie begins to unzip his forehead. Ganesh, Rose and Harriet are caught by Margaret Blaine, who unzips her forehead. In the room below, Asquith unzips his forehead, and Green - thanking the attendees for wearing their ID cards - uses a remote control to electrocute the experts including the Doctor. They announce themselves as the Slitheen.
- Doctor Who - Christopher Eccleston
- Rose Tyler - Billie Piper
- Jackie Tyler - Camille Coduri
- Spray Painter - Corey Doabe
- Policeman - Ceris Jones
- Reporter - Jack Tarlton
- Reporter - Lachele Carl
- Ru - Fiesta Mei Ling
- Bau - Basil Chung
- Matt Baker - As himself
- Andrew Marr - As himself
- General Asquith - Rupert Vansittart[note 1]
- Joseph Green - David Verrey
- Indra Ganesh - Navin Chowdhry
- Harriet Jones - Penelope Wilton
- Margaret Blaine - Annette Badland
- Doctor Sato - Naoko Mori
- Oliver Charles - Eric Potts
- Mickey Smith - Noel Clarke
- Alien - Jimmy Vee
- Strickland - Steve Speirs
- Slitheen - Elizabeth Fost, Paul Kasey, Alan Ruscoe
|Executive Producers Russell T Davies, Julie Gardner and Mal Young|
|Not every person who worked on this adventure was credited. The absence of a credit for a position doesn't necessarily mean the job wasn't required. The information above is based solely on observations of the actual end credits of the episodes as broadcast, and does not relay information from IMDB or other sources.|
- After the spaceship crashed in the Thames, residents of the Powell Estate put up a banner saying "ELLO E.T.", referring to the term made popular by the film E.T. the Extra-Terrestrial.
- Jackie claims that Billy Croot asked her out.
- Ballast is mentioned.
United Nations Intelligence Taskforce
- UNIT, the United Nations Intelligence Taskforce, is mentioned as being among the experts on aliens. The Doctor mentions having worked with them in the past, but that they wouldn't recognise him now.
- The Doctor claims to have participated in drinking contests with former Prime Minister David Lloyd George.
- The Doctor says he is nine hundred years old.
- The Doctor is forced to watch a clip of Blue Peter on television in which Matt Baker makes an alien ship cake.
Bad Wolf arc
- A boy spray paints the words "BAD WOLF" on the side of the TARDIS while it is parked on the Powell Estate.
- This episode had the working title Aliens of London Part One (World War Three being Part Two).
- This was the debut performance for Penelope Wilton as Harriet Jones. Her first line in the role was: "Excuse me! Harriet Jones, MP for Flydale North."
- The episode ends on a cliffhanger, the first since episode two of Survival. The story continues in World War Three. This is also the first occasion since Invasion of the Dinosaurs in which the first episode of a serial does not share its title with the second (Invasion of the Dinosaurs part one was titled Invasion).
- The official police poster is the first reference to the Powell Estate on the television series. In whole, the notice says, "Rose Tyler has been missing from her home on the Powell Estate since 6 March 2005. Rose is described as 19 years old, 5 feet 4 inches in height, slim build with shoulder-length blonde hair. Anyone with information regarding Rose should contact 0207 946000." The photograph used is one of Billie Piper herself, rather than one of Piper playing Rose. Several other home-made posters are seen on Jackie's table, including one with a banner headline saying "WHERE IS ROSE?".
- The scene where the pig-like "alien" is breaking through the metal door with Dr Sato watching in shock is reminiscent of an almost identical moment in the 1996 Doctor Who television movie, in which the newly-regenerated Eighth Doctor breaks through the metal door of the morgue, terrifying a hospital worker.
- The production team had intended to suggest that the murdered Prime Minister in this episode was current real-life incumbent Tony Blair. On the DVD commentary for the following episode, producer Phil Collinson explained that they had hired an actor to play the dead body on the understanding that the man was a Tony Blair lookalike. When the resemblance proved disappointing, they decided to avoid showing the body clearly. The suggestion that the body is Blair's remains in Harriet's line, "I'm hardly one of the babes", a reference to the large number of female Labour Party MPs who entered the House of Commons in Labour's 1997 general election victory, dubbed "Blair's Babes" by the British media. That Tony Blair was elected in the Doctor Who universe was confirmed in Rise of the Cybermen.
- According to Russell T Davies in Doctor Who Magazine, as well as Doctor Who Confidential, the decision to establish the Bad Wolf meme in the series did not occur until after the spur-of-the-moment decision to have the words "bad wolf" graffiti-painted on the TARDIS; subsequently Bad Wolf references were added to the scripts for most of the other Series 1 episodes, and notwithstanding a few minor or inferred references in the interim, returned in force in the Series 4 episodes Turn Left and Journey's End. With the origin of the meme established, the mystery that remains is exactly why the words "bad wolf" were chosen to be spray-painted on the TARDIS in the first place (as opposed to any other phrase).
- The TARDIS key began as an ordinary-looking Yale key, then changed during the Third Doctor's last season into a more alien looking one, which was also used for the first two seasons of the Fourth Doctor's tenure. It then reverted to the Yale key for the rest of the run of the original series. The alien key made one last appearance in the Doctor Who television movie. It has now returned to looking like an ordinary key, except that it starts to glow when the TARDIS is arriving.
- When the Doctor starts up the TARDIS to visit Albion Hospital, he plugs the sonic screwdriver into the console. The prop was originally meant to have a pair of "feet" under the black cap that would plug into the console but the idea was ultimately abandoned.
- The Doctor repeatedly refers to Mickey Smith as "Ricky". In Rise of the Cybermen, it is discovered that Ricky Smith is the alternative version of Mickey in Pete's World.
- This is the first episode to examine deeply the impact which the departure of a companion with the Doctor can have on those left behind. In this case, Rose's family believed her to have been murdered, and her boyfriend Mickey had become a suspect. The impact of a companion's travels with the Doctor on family and friends back on Earth becomes a recurring theme throughout the Russell T. Davies era.
- Beginning with this episode (broadcast in 2005, but set in 2006), most "modern day" Earth stories are said to take place approximately one year after the year in which they were broadcast. This has not been applied consistently. This displacement also applies to the later Torchwood and The Sarah Jane Adventures spinoff series. This would continue until The End of Time, at which point "modern day" stories once again synched up with the year in which they were aired, and explicitly followed for series 4 of The Sarah Jane Adventures and series 3 of Torchwood (and implied for SJA series 3).
- UNIT makes its first appearance on screen since TV: Battlefield. The Doctor spells out the acronym — United Nations Intelligence Taskforce. This would be the final on-screen use of this name, as by the time the full name of the organisation was uttered again in TV: The Sontaran Stratagem, it has been changed to Unified Intelligence Taskforce. (According to A Brief History of Time Travel the UN objected to the use of its name in the fictional organisation, even though it had been used as such since at least 1968.)  A modern day incarnation of UNIT also appeared around the same time in the 2004-2005 Big Finish audio series UNIT, which used Brigadier Lethbridge-Stewart as a central character.
- The story is the 700th episode of Doctor Who.
- Following the cliffhanger, the preview for World War Three was shown immediately afterwards. This was criticised as it showed the main characters had survived the life-threatening situations they'd just been shown in seconds previously. Following this, trailers for episodes with multiple-parts would be shown after the closing credits instead.
- 7.63 million viewers (UK final)
- Cardiff Royal Infirmary, Newport Road, Cardiff (exterior and interior of Albion Hospital)
- Westminster Bridge, Westminster, London (news report on the spaceship crash)
- Victoria Embankment, London (news report on the spaceship crash)
- Hensol castle, Glamorgan, Wales (interior of Downing Street)
- West Bute Street, Cardiff (traffic jam scene)
- Bute Street, Cardiff (when the Doctor and Rose are taken to Downing Street in the limo)
- Royal Society of Arts, John Adam Street, Westminster, London (exterior of Downing Street)
- Brandon Estate, Kennington, London (Powell Estate - TARDIS and rooftop scenes, Mickey's flat exterior, helicopter scene)
- Channel View Flats, Cardiff (Powell Estate stairwell)
- Unit Q2, Imperial Park, Imperial Way, Newport (studio filming)
- BBC Television Centre (Studio TC4), Shepherd's Bush, London (Blue Peter studio)
- HTV Wales Studios (Studio 1), Culvershouse Cross, Cardiff (pick-up studio work)
- The Model Unit Stage, Kendal Avenue, Acton (model effect filming) (all TCH 49)
- When Jackie Tyler slaps the Doctor, it is a clear miss.
- When the Slitheen ship is flying above London, what we see on the ground below the spaceship from close up shots does not match with what we see below the spaceship during long shots.
- When the alien announcements appear on the TV, they appear slightly blurry, as a screen would really be if taped. However, when the channels are changing, the image shown has no blur.
- The close-up shots of Big Ben as it is destroyed by an alien ship crashing into it, show the clock faces back-to-front. This is because the director wanted the ship to move across the frame the other way.
- Big Ben appears to keep running even after being struck by the spaceship. The time reads roughly 10:55 when the spaceship strikes it, but reads roughly 6:10 when the clock is later seen on a news broadcast.
- Around 26:32, when Mickey appears at the party, the BBC News broadcast in the background can be heard talking about how, "with no head of state", "it falls to Harriet Jones to form an emergency government". This didn't happen until the subsequent episode.
- At one point, a news reporter is standing by the Thames, citing an alien body being found; however, after the channel is changed to Blue Peter for around 30 seconds, once it's changed back to the news, the same reporter is now standing in an entirely different location.
- When Jackie has called the police to her flat the Doctor alternates between shots, sometimes having his hands in his pockets, other times having his arms crossed.
- In one close-up of a photographer taking pictures outside 10 Downing Street the scene is reversed to provide an opposite angle. You can see the "Nikon" nameplate is reversed on the front of the camera.
- When the Doctor is kneeling over the Slitheen's engineered Pig Mutant, the seam of the plastic mould of the costume is clearly seen.
- In the close-up shots of the Pig Mutant, the Pig is clearly seen still breathing.
- When Rose enters the TARDIS after the Doctor returns from the hospital, the Powell Estate is clearly not outside the door; it is simply a black background.
- When the TARDIS lands in Albion Hospital, the "BAD WOLF" graffiti has disappeared, only to return when the TARDIS lands back at the Powell Estate.
- When the Slitheen is climbing out of its skin suit for the first time in front of the experts, while flailing, its left arm would've hit Green, yet it simply passes through him.
- When Mickey pulls the TARDIS door shut, the 'Pull to Open' door comes open, but in every following shot, it's closed again.
- Rose mentions going to the year 5,000,000,000. (TV: The End of the World)
- Dr Sato is later revealed to be Toshiko Sato. (TV: Everything Changes) She was working undercover for Torchwood during this incident and in fact her colleague Dr Owen Harper was originally to have done the autopsy, but he had a hangover and was just recently appointed to the team. (TV: Exit Wounds)
- Mickey has researched the Doctor and learned that he worked for UNIT. (TV: Spearhead from Space) The Doctor claims they won't recognise him, as he's "changed a lot since the old days".
- The TARDIS was previously defaced with chalk scribblings (TV: The Time Warrior, The Leisure Hive), graffiti, (TV: Paradise Towers) and pink paint. (TV:The Happiness Patrol)
- The Doctor visits Albion Hospital again in 1941. (TV: The Empty Child, The Doctor Dances)
- The crash (and the events that follow) are seen by Elton Pope from an entirely different point of view. (TV: Love & Monsters)
- The Doctor tells Rose he is 900 years old, but he had previously (TV: Time and the Rani) given his age as older. (See The Doctor's age.)
- Margaret comments that "we" have to fix gas exchange. A few years later, slimline flesh suits with no gas exchange will be used by the Slitheen. (TV: The Lost Boy)
- During his first incarnation, the Doctor previously visited 10 Downing Street in July 1900 in the company of Steven Taylor and Vicki Pallister. (AUDIO: Upstairs)
Home video releases
- Aliens of London together with World War Three and Dalek were released in a collection in both DVD and UMD format in the UK in 2005. The DVD version was released in North American in 2006.
- This was also released as a vanilla release along with the next two episodes.
- It was also released as part of the Series 1 DVD box set.
- This was also released with Issue 2 of the Doctor Who DVD Files.
- BBC - Doctor Who - Episode Guide - Aliens of London
- Aliens of London at Shannon Sullivan's A Brief History of Time (Travel)
- The Discontinuity Guide to: Aliens of London at The Whoniverse
- Aliens of London at The Locations Guide
- Five-Minute Aliens of London — Parody version