The TARDIS brings Rose and a comatose Doctor back to London on Christmas Eve. As Jackie and Mickey help Rose care for the ailing Time Lord, a recently-launched British space probe attracts the attention of the warlike Sycorax. Prime Minister Harriet Jones and UNIT staunchly defy the aliens -- until the Sycorax use blood technology to take control of fully one-third of the planet's population, threatening the lives of two billion people unless they are declared masters of the Earth.

The dust had barely settled on the premiere of the first new Doctor Who season in 16 years when, on March 29th, 2005, BBC Head of Drama Jane Tranter revealed that the programme would be returning for a second slate of episodes, to be preceded by Doctor Who's first-ever Christmas special. This was immediately assigned to executive producer Russell T Davies, who labelled it The Christmas Invasion. With the Ninth Doctor, Christopher Eccleston, having informed the production team by the end of 2004 that he would be leaving Doctor Who after his inaugural season, Davies was aware that The Christmas Invasion would be the first full-length adventure for Eccleston's successor.

In plotting the special, Davies was adamant that the story actually be set at Christmastime. He wanted a threat on an epic scale to justify the full one-hour running length he was being accorded, contrasted with the simple yuletide festivities of Rose Tyler's home. Davies was also eager to include elements of the first season to reassure viewers discomfited by the sudden change of lead actor; to this end, he and producer Phil Collinson quickly secured the services of Penelope Wilton. Wilton had played MP Harriet Jones in Aliens Of London / World War Three, and would now see her character promoted to Prime Minister.

Davies briefly considered destroying Big Ben again in The Christmas Invasion -- as had happened in Harriet's debut story -- but later decided to depict the monument as being under repair. The adventure's alien adversaries came to be called the Sycorax, originally the name of Caliban's mother in William Shakespeare's The Tempest. Davies also included a substantial role for the United Nations Intelligence Taskforce (UNIT), an organisation which had first appeared in Doctor Who in 1968's The Invasion and which had been briefly glimpsed in Aliens Of London / World War Three. However, because the genuine United Nations had reacted unfavourably to a UNIT website set up by the BBC to help promote the first season of Doctor Who, Davies referred to the Taskforce by their acronym only. Also referenced in the script were Davies' nephew and niece -- Jonathan and Catherine -- whose names he gave to the blood-controlled children.

On April 16th, the British press carried the news that David Tennant had been cast as the new star of Doctor Who. By this time, work was under way to flesh out the look and demeanour of the Tenth Doctor. Tennant was in favour of a costume featuring a long coat and many pockets, and it was felt that his incarnation should have a less aggressive appearance than Eccleston's Doctor. Costume designer Louise Page eventually assembled an outfit highlighted by a grey pinstripe suit; Page also wanted to dress Tennant in Japanese army boots, but the actor preferred his own plimsolls to give the Doctor a more casual air. Davies, meanwhile, had written The Christmas Invasion largely with Eccleston's Doctor in mind -- the idea being that the characteristics of the Tenth Doctor would emerge more through Tennant's portrayal than from scripted lines. It was agreed that Tennant would speak in an English accent rather than his native Scottish brogue, with a scripted line (later cut in editing) explaining that the new Doctor's speech patterns were based on Rose's.

As the revived Doctor Who's extremely successful first season drew to a close, the press was aflutter with rumours that Billie Piper would be absent from some or all of the second run. These reports were finally quelled on June 15th, when it was confirmed by the BBC that Piper would appear as Rose in both The Christmas Invasion and all of the second season. What was not revealed at this point was that Piper had informed the production team that she would be departing Doctor Who once these fourteen episodes were completed, prior to the already-announced 2006 Christmas special.

The first season had been loosely connected by the words “Bad Wolf”, which had appeared in virtually every episode in some form, building to the revelation in the finale, The Parting Of The Ways, that they were a message scattered through time and space by a vortex-powered Rose Tyler. Davies sought to repeat the experiment in the new season, and began developing the notion of the Torchwood Institute, first mentioned in Bad Wolf. (The name “Torchwood” was actually an anagram of “Doctor Who”, coined to conceal the identity of footage from episodes which had not yet been broadcast.) Subsequently, Davies was asked to turn the Torchwood concept into a spinoff series, to star John Barrowman as former companion Jack Harkness. Although he did not anticipate mentions of Torchwood arising as frequently in the second season as Bad Wolf had in the first, Davies was nonetheless eager to tease the concept in The Christmas Invasion, and this became a prominent element of his script.

Although it would air several months before the beginning of the 2006 season, The Christmas Invasion was made as part of the same production schedule, joining New Earth and School Reunion in comprising Block One. The director was James Hawes, who had helmed The Empty Child / The Doctor Dances the year before.

Filming for The Christmas Invasion began in Newport on July 22nd, when material which would appear on TV screens (such as Harriet's address to the nation) was recorded at Tredegar House and Unit Q2, the dedicated Doctor Who studio space. This was followed by a week in London. Most of the material to be filmed was set on the Powell estate, and was captured at either Wallis House in Brentford (July 25th) or Brandon Estate in Kennington (July 26th, 27th and 29th). The exception was the 28th, when the Doctor Who team became the first television drama to be granted permission to film atop the Tower of London. Recording was enlivened on the last London day by the presence on-set of Peter Davison, who had played the Fifth Doctor; Davison had earlier sent well-wishes to Tennant as he embarked on his first Doctor Who story, as had Fourth Doctor Tom Baker.

More sequences on the Tylers' estate were filmed upon the team's return to Cardiff on August 1st, this time at Loudoun Square in Gabalfa. The 2nd was a studio day, split across the TARDIS set at Unit Q2 and the Tyler flat, erected at HTV Studio 1 in Culverhouse Cross. Recording at HTV continued through August 5th. The 8th was divided between scenes involving the blood-controlled masses ascending fire escapes (actually Baltic House in Cardiff Bay and the British Gas Building), and those set at the Christmas market, at The Hayes. For the latter, the Cardiff city authorities agreed to erect their yuletide decorations, while Howell's department store once again posed as Henrik's, as it had done in Rose the year before. August 9th was also spent at The Hayes, as well as Broadstairs Road in Leckwith for more scenes of the brainwashed Londoners.

August 10th then saw cast and crew shift to Clearwell Caves, near Coleford in Gloucestershire. Unusually, this site was chosen to represent the interior of the Sycorax vessel, reflecting Davies' desire to get away from traditional metallic spacecraft. Work at the caverns continued until the 13th. Unfortunately, the difficulty of transporting equipment into and out of Clearwell Caves meant that a planned motion-capture shot, tracking into the TARDIS console room when the revived Doctor throws open the doors, had to be abandoned.

One more day at Clearwell Caves was scheduled for August 15th, the same day that the shots of Mickey in Clancy's Garage were captured at Brian Cox Motor Engineering in Elwood. On the 16th, the swordfight between the Doctor and the Sycorax Leader on the wing of the spaceship was performed at the docks on Barry Island -- the only location Hawes could find which offered him an unobstructed view of the horizon. The next three days were spent at Cardiff's Millennium Stadium, for scenes in the UNIT control centre. Davies specified in his script that this was actually the headquarters of the British Rocket Group, an homage to Nigel Kneale's pioneering Quatermass serials of the Fifties. Green screen work then followed at HTV on August 22nd.

Model shooting took place at the BBC Model Unit in London on September 6th and 7th. Inserts of the TARDIS console room were recorded at Unit Q2 on the 8th, while that of the Santa mask rolling on the ground was performed in the Unit Q2 car park on September 22nd. Tennant filmed the scene in the TARDIS wardrobe at Newport on October 8th, on a redressed version of the console room set. Amongst the many costumes seen in this sequence were a shirt reminiscent of Tennant's starring role in the Russell T Davies-produced Casanova, and Hogwarts robes, alluding to his forthcoming appearance in the feature film Harry Potter and The Goblet Of Fire. Also visible were each of the previous Doctor's outfits, with the Fourth Doctor's multi-coloured scarf actually belonging to producer Phil Collinson (for whom it had been specially knit by his great-aunt). Several other pick-up shots were completed at Unit Q2 on this day and on November 3rd. The last material recorded for The Christmas Invasion was the inserts of the newsreaders, recorded in the C2 News Studio at BBC Broadcasting House in Llandaff.

Originally, Davies had hoped to include scenes of mind-controlled humans near the pyramids in Cairo and on Sydney Harbour Bridge to emphasise the fact that the Sycorax were a global threat, but neither could be realised convincingly and were dropped. The finishing touch was placed on The Christmas Invasion on November 14th, when composer Murray Gold recorded the story's incidental music and a new version of his theme arrangement with the BBC National Orchestra of Wales.

Soon thereafter, a wave of publicity began for The Christmas Invasion, most notably a specially-created cover of the Radio Times -- marking the first time in sixteen years that the magazine had opted to spotlight a single programme on its holiday issue. The Christmas Invasion was granted a prestigious Christmas Day timeslot, making it just the second Doctor Who episode broadcast on December 25th (the other being part seven of The Daleks' Master Plan in 1965). And Tennant's first full-length adventure lived up to the hype, as he became just the second Doctor (after Eccleston) to debut in the Top Ten programmes for the week. For the Doctor Who production office, it was a very merry Christmas indeed.

• Outpost Gallifrey: New Doctor Who Series News Page, edited by Shaun Lyon.
• Doctor Who Magazine Special Edition #14, 9th November 2006, “Episode X: The Christmas Invasion” by Andrew Pixley, Panini Publishing Ltd.