The Doctor thinks he has brought Rose home twelve hours after she left, only to discover that it has instead been twelve months. But before they can explain the situation to a panic-stricken Jackie and a wary Mickey, a spaceship crashlands in the heart of London. Much to the Doctor's astonishment, its pilot appears to be a pig. Meanwhile, lowly MP Harriet Jones discovers that the Prime Minister has gone missing, and some of the senior administrators in Whitehall are behaving very strangely. These events are all part of a deception engineered by the alien Slitheen, who have infiltrated 10 Downing Street itself.

During the twentieth century, Doctor Who stories had almost always been comprised of two or more episodes; indeed, for most of its existence, the programme had been made by the Drama Serials department. Apart from the movie-length The Five Doctors in 1983 and the co-produced Doctor Who (1996), the only standalone adventure had been 1965's Mission To The Unknown -- and it was effectively a prologue for the twelve-part The Daleks' Master Plan, which started airing a month later. For Doctor Who's 2005 revival, however, executive producer Russell T Davies decided to take advantage of the faster pace of modern television drama, as well as the fact that the programme would now consist of forty-five-minute episodes, as opposed to the twenty-five-minute broadcasts which had been typical. This would allow him to emulate the pattern of many American genre series, with most installments telling self-contained stories. Two-part adventures would serve as special tentpole events during the season, while also conjuring the nostalgia of a traditional Doctor Who cliffhanger.

In his pitch document of December 8th, 2003, Davies indicated that the first of Season Twenty-Seven's two-part stories would consist of episodes entitled Aliens Of London and “10 Downing Street”, which would air fourth and fifth. They 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 from Virgin Publishing. Davies planned for the serial to focus on Rose's relationship with her mum, at this point still called Judy rather than Jackie.

To emulate Quatermass And The Pit, the “alien” spacecraft was originally found buried in a construction lot

Davies began writing Aliens Of London / “10 Downing Street” around April 2004, taking inspiration from disparate sources. For the opening episode, he sought to emulate 1958's seminal science-fiction serial Quatermass and The Pit. As such, it originally featured the discovery of a buried “alien” spacecraft in a construction lot in Tottenham. As Davies began to realise what could be achieved with computer-generated effects, however, he instead added the ship's destructive crashlanding, providing greater scale to the narrative. The Whitehall setting of the concluding installment, on the other hand, had a far more contemporary inspiration. This was Girls Aloud's video for their 2003 cover Jump, which featured scenes of the Prime Minister -- played by Hugh Grant -- at 10 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 inversion of Davies' original notion. His early ideas saw Rose astonished to find that the TARDIS had brought her home just an hour after her departure, rather than a year too late. Davies also decided to delve into Doctor Who's past with the inclusion of the United Nations Intelligence Taskforce (UNIT), which had been introduced in 1968's The Invasion and had been an integral part of the programme throughout the early Seventies. 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.

As he began drafting his scripts, Davies became concerned that Aliens Of London / “10 Downing Street” lacked enough elements of fun. As such, he developed the character of Harriet Jones. She came about when Davies was discussing actress Penelope Wilton -- who had appeared in his sitcom Bob & Rose -- with producer Phil Collinson, and Collinson suggested developing a role for Wilton in Doctor Who. With production looming, Davies also decided to take advantage of the fact that the inclusion of the spaceship crash meant that he could feature a live “alien” in the story; originally, the vessel had been inhabited by a corpse which turned out to be nothing more than a shank of beef. Executive producer Julie Gardner was unhappy with the notion of the “space pig”, but she decided to trust Davies' instincts. Until a very late stage, the pig was to be dressed only in a cloak, but Davies and costume designer Lucinda Wright finally decided that it should wear a Fifties-style spacesuit.

Meanwhile, a minor addition to the scripts would wind up having major implications for Season Twenty-Seven as a whole. Keen to add realistic touches to his depiction of life on the Powell Estate where the Tylers had their flat, Davies included a boy who spray-painted “bad dog” on the side of the TARDIS. This was later amended to “bad wolf”, and would eventually help inspire the story arc which tied the 2005 episodes together. Davies also had to include a scene of the boy being made to clean up his graffiti, in accordance with BBC standards.

Russell T Davies began to consider ways to bring the Slitheen disguised as Margaret Blaine back to Doctor Who

Davies completed his first draft of the serial on May 17th. By this time, it was known that the two episodes would be made as Block One alongside the season premiere, Rose, since they shared a contemporary setting and both stories featured Jackie Tyler and Mickey Smith. Consequently, the director would be Keith Boak. During rehearsals, Davies was very impressed with the performance given by Annette Badland, who was playing the Slitheen disguised as Margaret Blaine. He soon began to consider ways to bring the character back to Doctor Who.

The very first scene filmed for the revival of Doctor Who was from Aliens Of London. It was taped on July 18th at the Cardiff Royal Infirmary, which was posing as Albion Hospital. Later the same day, Christopher Eccleston made his debut as the Ninth Doctor, hurrying in pursuit of the space pig. Also present was Naoko Mori, in the small role of Dr Sato; like Badland, Davies made note of her talent and was keen to work with her again in the future. Cast and crew remained at the Infirmary on the 19th.

Production then moved to London for a week. Having been denied permission to record in front of the real 10 Downing Street, the team approached the sketch show Little Britain for advice in finding a substitute location. As a result, July 25th was spent in front of the Royal Society of Arts on John Adam Street in Westminster. Here, BBC News political editor Andrew Marr agreed to make a cameo appearance for material originally written for an anonymous reporter. Recording on July 26th began with the crashing spaceship and the resulting news reports along the Victoria Embankment in Westminster. Unfortunately, work was delayed by overzealous anti-terrorism officers whose suspicions were aroused by the presence of ersatz policemen in proximity to the Houses of Parliament. The workday ended at dawn the next morning, with the sequence of Harriet Jones addressing the reporters in Whitehall. Activity from July 27th to 30th centred on the Brandon Estate in Kennington, for scenes set on the Powell Estate.

Cast and crew then returned to Wales, with the ruins of Downing Street erected on Lower Dock Street in Newport on August 3rd. The major location for the story was Hensol Castle in Pendoylan, an edifice originally built in the fifteenth century which had undergone numerous periods of extensive renovation. It provided the interior of 10 Downing Street from August 4th to 6th, 8th to 13th, and 16th to 19th. August 4th was also the day that the BBC announced that Penelope Wilton would be appearing in Doctor Who. In pre-production, there had been considerable discussion about how to realise the Slitheen, with Boak in particular pushing for a combination of practical costumes and computer-generated imagery. At Hensol Castle, however, the limitations of the costumes became apparent, and it was later found that the digital Slitheen matched imperfectly with their live-action counterparts. As a result, the production team agreed to avoid such a hybrid approach in the future. There was also disappointment that extra Roderick Mair, who had been hired to play the murdered Prime Minister, did not better resemble the office's current incumbent, Tony Blair. As a result, Boak kept shots of Mair to a minimum.

August 20th was the first day of production at Doctor Who's new studio home, Unit Q2 in Newport, during which scenes in Jackie's bedroom were taped. On the 21st, footage from the fictional AMNN news channel was filmed at BBC Broadcasting House in Cardiff. Lachele Carl made the first of several appearances as the American newsreader; her previous credits included episodes of Star Cops and Grange Hill, and a small role in the 1989 blockbuster movie Batman. Several days at Unit Q2 followed, with Boak recording material in the Tylers' flat on August 26th and 27th, at Heathrow Airport on the 28th, at various places on the Powell Estate on the 30th, in Mickey's apartment on the 31st and September 1st, and in the TARDIS on the 2nd and 3rd.

Unfortunately, the shoot had gotten progressively behind the schedule, and it was now realised that there had been an overestimation of the pace of production, given the enormous complexities of making Doctor Who. This meant that an extra week would have to be allocated to Block One, starting at Unit Q2 on September 6th with more footage in Jackie's kitchen and outside Mickey's door. On the 8th, Rose ran up the Powell Estate stairwell at Channel View Flats in Cardiff. Boak's team returned to the Cardiff Royal Infirmary on September 9th for further scenes at Albion Hospital. Then it was back to BBC Broadcasting House on September 11th for shots of the ocean -- to which the missile launch would be added in post-production -- and the elevator shaft. Model filming was conducted from September 14th to 16th at the BBC Model Unit Stage in Acton, London.

A more unusual recording session was held three weeks later. Throughout its twentieth-century run, the Doctor Who production office had enjoyed an excellent relationship with the venerable children's programme Blue Peter. Davies decided to pay tribute to this tradition in Aliens Of London and, on May 4th, he secured the cooperation of Richard Marson, the editor of Blue Peter and a former contributor to Doctor Who Magazine. So it was that, on October 4th, a clip of Blue Peter presenter Matt Baker making “spaceship cakes” was recorded at BBC Television Centre Studio 4 in White City, London.

The rapid tempo of the episodes meant that both Aliens Of London and “10 Downing Street” were under-running

During editing, it was discovered that the rapid tempo of the episodes meant that both Aliens Of London and “10 Downing Street” were under-running. As a result, Davies wrote new material which could be taped during Block Three. Most notably, the end of the concluding installment was substantially extended; originally, after surviving the destruction of 10 Downing Street, there had been just a short sequence of the Doctor and Rose leaving the Powell Estate to head back to the TARDIS. On November 9th, extra TARDIS material was recorded at Unit Q2. On the 10th, the interior of the Tylers' flat was erected at HTV Wales Studios in Cardiff; scenes filmed there included Jackie interrogating Rose about her whereabouts during her missing year. The Slitheen picture appearing on Rose's phone was taped at HTV Wales Studios on November 22nd. Finally, on the 24th, the Doctor and Rose watched the traffic jam at West Bute Street in Cardiff, and the time travellers were chauffeured to 10 Downing Street along Bute Street, Loudoun Square and Lloyd George Avenue.

There had been some uncertainty as to whether each episode of Season Twenty-Seven would be given an on-screen title and, in the case of the two-part stories, whether each installment would have an individual name -- as had been the case on Doctor Who between 1963 and 1966 -- or be grouped together under an umbrella title. Indeed, some thought was given to broadcasting the Slitheen adventure as Aliens Of London Episode One and Episode Two. By mid-January 2005, however, it was agreed that the first installment would be called Aliens Of London, while the second would be labelled “World War 3”, subsequently amended to World War Three.

As publicity surrounding the revived Doctor Who series ramped up, the BBC was contacted by the United Nations. They were no longer comfortable with their association with a fictional alien-hunting organisation, and asked for changes to the UNIT acronym. Consequently, when the BBC launched a UNIT website to build interest in Aliens Of London / World War Three, it was described as belonging to the UNified Intelligence Taskforce. The amended name would be employed in all future television references to UNIT.

The weeks leading up to the broadcast of Aliens Of London / World War Three saw 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 furore over the revelation that Eccleston would not be returning to the programme. In the wake of considerable press speculation, the BBC announced on April 16th 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. That evening, Aliens Of London was transmitted, and marked the last time that Doctor Who led into The National Lottery: Jet Set. The next week, World War Three was followed on BBC1 by a new quiz show called The National Lottery: Come And Have A Go.

• Doctor Who Magazine Special Edition #11, 31st August 2005, “Fact File: Aliens Of London / World War Three” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #49, 2017, “Story 160: Aliens Of London / World War Three”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Inside Story by Gary Russell (2006), BBC Books.