The Doctor tries to bring Rose home to see her mother, only to have the TARDIS inadvertently arrive a year late. As Rose tries to deal with a panic-stricken Jackie and a wary Mickey, a spaceship crashlands in the heart of London. But lowly MP Harriet Jones discovers that the incident is just a diversion concocted by the alien Slitheen, who have already infiltrated 10 Downing Street itself.

From its conception, Doctor Who had been envisioned as a series of multi-episode serials. Apart from movie-length specials, only one episode -- Mission To The Unknown -- had stood alone (and even that really only served as a prologue to the twelve-part The Daleks' Master Plan). However, as executive producer Russell T Davies began developing the new Doctor Who series in 2003, he quickly decided that the majority of episodes would be self-contained (albeit twice as long as a typical original-series Doctor Who broadcast), relying on the faster pacing of modern television drama to tell a fully-developed story.

Nonetheless, Davies was keen to include a handful of two-part adventures in the mix, both to provide the space to tell larger, “event” stories and to retain the nostalgia feel of the traditional Doctor Who cliffhanger. In his pitch document, prepared in the fall of 2003, Davies indicated that the first of these would consist of episodes entitled Aliens Of London and “10 Downing Street”. These were to feature the first major new monster race in the revived series: the Slitheen clan from the planet Raxacoricofallapatorius. The idea of having a family as antagonists was inspired by the Aubertides, the villains of Paul Cornell's 1995 Doctor Who: The New Adventures novel Human Nature.

The content of the two installments were inspired by disparate sources. For Aliens Of London, Davies sought to emulate 1958's seminal British science-fiction serial Quatermass and The Pit. Originally, this episode featured the discovery of a buried “alien” spacecraft in a construction lot in Tottenham. It was only as production loomed that Davies began to realise what could be achieved with CGI, and so rewrote his scripts to provide greater scope for the introduction of the “alien”, with the ship's destructive crashlanding now part of the narrative. Davies also elected to feature a live “alien” in the story, rather than just a corpse which would turn out to be nothing more than a shank of beef. The eponymous setting of “10 Downing Street”, on the other hand, had a far more contemporary inspiration: the Girls Aloud video for Jump, which featured scenes of the Prime Minister (played by Hugh Grant) in Downing Street from the film Love, Actually.

There were further changes to the storyline as Davies developed it from its initial pitch form. In particular, Jackie was meant to accompany the Doctor and Rose to 10 Downing Street, while the start of the story was a complete reversal of Davies' initial concept. Originally, the TARDIS brought Rose back home just minutes after she departed, much to her surprise -- rather than arriving a year too late. Davies also decided to delve into Doctor Who's past with the inclusion of a cameo appearance by members of the United Nations Intelligence Taskforce (UNIT), a recurring element of the original Doctor Who series beginning with The Invasion in 1969. The name of one of the personnel was given as Colonel Muriel Frost, a reference to a character created for Doctor Who Magazine's comic strip in 1991.

Although Aliens Of London and “10 Downing Street” were always meant to be the fourth and fifth episodes broadcast, they were paired with Rose as part of the first production block because both stories shared a contemporary setting. As such, the adventure was helmed by Keith Boak. Indeed, the first scene filmed for the new Doctor Who series was from Aliens Of London. This took place on July 18th, 2004 at the Cardiff Royal Infirmary, posing as Albion Hospital; work continued there the following day.

Production then moved to London for a week. A replica of the 10 Downing Street exterior was erected on John Adams Street on the 25th. The 26th began with material involving the crashed spaceship at Victoria Embankment; unfortunately, work was delayed by overzealous anti-terrorist officers whose suspicions were aroused by the shoot's proximity to the Houses of Parliament. The workday ended at dawn the next morning, with the sequence of Harriet Jones addressing the reporters on Westminster Street. The following four days centered around Brandon Estate in Kennington, for scenes set around the Tylers' and Mickey's flats. Cast and crew then returned to Wales, with taping on August 3rd involving material set in the lift (at the University Hospital of Wales in Cardiff), and in the ruins of Downing Street (on Lower Dock Street in Newport).

The major location for the story was Hensol Castle in Hensol, which posed as the interior of 10 Downing Street. Work took place there from August 4th to 6th, 8th to 13th, and 16th to 19th. Next there followed several studio days for material set in Jackie's and Mickey's apartments, as well as the TARDIS interior. These took place at Unit Q2 in Newport, and occurred on August 20th, 26th to 31st (except the 29th), September 1st to 3rd, and finally the 6th. During this period, footage of the newsreaders was recorded at BBC Broadcasting House in Llandaff on August 21st.

Recording was intended to be completed by this stage, but the complexity of the first production block had caused Boak to fall badly behind schedule. Consequently, additional time was allocated to the story. On September 8th, Rose running up the estate stairwell was filmed at Channel View Flats in Cardiff, while more scenes in Albion Hospital were enacted at the Royal Infirmary the next day. The production returned to BBC Broadcasting House on September 11th for shots of the elevator shaft and the ocean (for the missile's flight). Model filming was conducted from the 14th to the 16th at the BBC Model Unit Stage in London.

A more unusual recording session was held in October. For many years, the Doctor Who production office had had an excellent relationship with the long-running children's programme Blue Peter. Davies decided to pay tribute to this tradition in Aliens Of London, and on October 4th, a clip of Blue Peter presenter Matt Baker baking “spaceship cakes” was recorded in BBC Television Centre Studio 4 for insertion into the episode. This took place during the main taping for the second production block (The End Of The World and The Unquiet Dead). Baker was not the only celebrity cameo appearance in the adventure; the BBC's Political Editor, Andrew Marr, had earlier taped an appearance in a role originally scripted as simply “Reporter #2”.

Shortly thereafter, it was discovered that both Aliens Of London and “10 Downing Street” were underrunning, and so Davies wrote new material which was taped in November during Block Three (Dalek and Father's Day). On the 9th, extra TARDIS scenes (most notably of the Doctor tracing the spaceship's course) were taped in Unit Q2. The next day, the interior of Jackie's flat was erected at HTV Wales' Studio 1 in Culvershouse Cross; scenes taped here included Jackie interrogating Rose about her whereabouts during her missing year, and the exchanges between mother and daughter from the story's concluding moments. The Slitheen picture appearing on Rose's phone was taped at Culvershouse Cross on the 22nd. On the 24th, material of the Doctor and Rose being caught in the traffic jam, and later conversing as they are chauffeured to 10 Downing Street, was filmed on various Cardiff streets. Finally, numerous inserts were taped at Unit Q2 on the 26th. In March, the title of the second installment became World War Three.

The weeks leading up to the broadcast of Aliens Of London and World War Three were a tempest of publicity for Doctor Who, from the anticipation of the debut of Rose, to the excitement over the renewal of the series for a second season, and culminating with the furour over the revelation that Christopher Eccleston would not be returning to the programme. In the wake of considerable press speculation, April 16th -- the day of Aliens Of London's transmission -- coincided with the announcement that the role of the Tenth Doctor had gone to rising star David Tennant (who, despite media reports to the contrary, had actually signed his contract several weeks earlier).

A longtime Doctor Who fan, Tennant (born David MacDonald) had appeared in several Doctor Who audio dramas from Big Finish Productions, including Colditz as Feldwebel Kurtz and Medicinal Purposes as Daft Jamie. He had also played the Caretaker in the 2003 BBC webcast Scream Of The Shalka (starring Richard E Grant as a putative Ninth Doctor) and just weeks earlier had been heard narrating the BBC1 documentary Doctor Who: A New Dimension. Tennant had recently worked with Davies on the well-received Casanova, and with The Unquiet Dead writer Mark Gatiss on the 2005 version of The Quatermass Experiment. He had also begun to garner several notable film credits, including Harry Potter and The Goblet Of Fire and Bright Young Things.

• Outpost Gallifrey: New Doctor Who Series News Page, edited by Shaun Lyon.
• Doctor Who Magazine Special Edition #11, 31st August 2005, “Fact File: Aliens Of London/World War Three” by Andrew Pixley, Panini Publishing Ltd.