On Christmas Eve, the TARDIS returns to the Powell Estate, where the newly-regenerated Doctor collapses. Jackie and Mickey help Rose care for the ailing Time Lord, who attracts unwanted attention in the form of menacing robotic Santas and a lethal Christmas tree. Meanwhile, a British space probe is captured by the warlike Sycorax as it makes its final approach to Mars, prompting a defiant response from Prime Minister Harriet Jones and UNIT. But when the Sycorax take control of one-third of the Earth's population, Harriet realises that only the Doctor can save them... if he's ever able to emerge from his comatose state.

With just weeks to go before its television premiere, the revival of Doctor Who reached a major crossroads in early 2005. Towards the end of the previous year, lead actor Christopher Eccleston had informed executive producers Russell T Davies and Julie Gardner that he would not be returning as the Ninth Doctor following the end of his introductory season. Eccleston's relationship with his bosses had soured soon after the start of production during the preceding July. As a result, Davies and Gardner offered the role of the Tenth Doctor to David Tennant, with whom they had worked on the comedy-drama Casanova. At that stage, however, there was no certainty that Doctor Who would continue beyond its season finale in June.

That changed on February 1st, when Davies and Gardner met with Jane Tranter, the BBC's Controller of Drama Commissioning. Tranter had been instrumental in the decision to relaunch Doctor Who in the first place. Now she gave the show and its production team a huge vote of confidence by confirming that the BBC wanted not only a twenty-eighth season in 2006, but also a one-hour Christmas special for December 2005. Previously, the lone holiday-themed Doctor Who episode had been The Feast Of Steven, the seventh installment of The Daleks' Master Plan, which aired on Christmas Day 1965; the BBC had subsequently avoided scheduling Doctor Who at Christmas. Furthermore, the only feature-length specials which had ever been made for the show were 1983's twentieth-anniversary celebration, The Five Doctors, and Doctor Who (1996), a failed attempt to revive Doctor Who as an American co-production.

Russell T Davies had tentatively planned to open the 2006 season with a full-scale invasion of Earth

The new set of episodes was formally commissioned on February 2nd. Some confusion initially resulted: whereas Season Twenty-Seven had consisted of thirteen episodes, Davies assumed that the addition of a Christmas special would reduce Season Twenty-Eight to twelve installments. In fact, Tranter intended the holiday programme to bolster the episode count to fourteen. However, this would force the production team to rethink the structure of the season, since it would be challenging to make that many shows within the planned recording schedule. Davies had already given some thought to potential storylines for the Tenth Doctor. One scenario he had not explored during Season Twenty-Seven was a full-scale invasion of Earth, and he had tentatively planned to open the new run with such a narrative. Now he decided that this idea would work better within the extended running time of the Christmas special. Davies was adamant that the episode should actually be set during the holiday season, and so he dubbed it The Christmas Invasion.

To reassure viewers discomfited by the change of lead actor, Davies was keen for The Christmas Invasion to include familiar elements from Season Twenty-Seven. The epic alien threat would contrast with the Doctor's involvement in the simple yuletide festivities celebrated by Rose Tyler, her mother Jackie, and her sometime-boyfriend Mickey Smith. Furthermore, Davies and producer Phil Collinson quickly secured the services of Penelope Wilton, who had played MP Harriet Jones in Aliens Of London / World War Three. Wilton would now see her character promoted to Prime Minister -- as the Doctor had predicted in the earlier story -- but would feel compelled to make difficult decisions which would lead to her downfall. Collinson was fond of the character, and he argued against her downbeat fate, which was partly fuelled by the sense of betrayal Davies felt towards the Labour government of Prime Minister Tony Blair. However, it would help set up both the Torchwood spin-off series which Davies was planning, and a possible story arc for 2007 in which the next Prime Minister was revealed to be the Doctor's Time Lord arch-nemesis, the Master.

The Guinevere 1 probe was inspired by the Beagle 2, a British-made spacecraft which landed on the surface of Mars on Christmas Day 2003. Sadly, the malfunction of its solar panels prevented the Beagle 2 from establishing contact with its operators on Earth, and the mission was deemed a failure. Davies also remembered the first pictures sent from Mars by the Viking 1 lander in July 1976, and he wanted the Guinevere 1 to transmit something more exciting than the empty, bleak Martian landscape. This suggested the monstrous Sycorax, who were named after the mother of the grotesque Caliban in William Shakespeare's fantasy The Tempest. At one stage, the Sycorax were assisted by an ill-fated civil servant called Sir Aubrey. Having destroyed Big Ben in Aliens Of London, Davies considered a scene in which the Sycorax spaceship decimated the newly-rebuilt structure -- leading to Sir Aubrey's death beneath the falling rubble. Instead, he decided to demolish a much more modern London landmark: the Gherkin, opened in 2004 as the British headquarters of Swiss Re.

Russell T Davies gave the blood-controlled children the names of his nephew and niece, Jonathan and Catrin

The Christmas Invasion included a substantial role for the United Nations Intelligence Taskforce (UNIT), an organisation which had first appeared in 1968's The Invasion and which had played a small role 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 Season Twenty-Seven, Davies decided to refer to the Taskforce by the UNIT acronym only. Also mentioned in the script were Davies' nephew and niece, Jonathan and Catrin, whose names he gave to the blood-controlled children. He made little effort to differentiate the new Doctor's dialogue from that of his predecessor, anticipating that the Tenth Doctor's characteristics would emerge more through Tennant's portrayal than from scripted lines.

In the early spring, Tennant formally committed to playing the Doctor. Eager to avoid a repeat of Eccleston's brief tenure, the BBC contracted him for three seasons. Tennant was soon working with costume designer Louise Page to develop the image of the Tenth Doctor. The actor wanted a long coat and many pockets, and it was felt that his incarnation should have a less aggressive appearance than Eccleston's. Page assembled an outfit highlighted by a brown pinstripe suit; she also wanted to dress Tennant in Japanese army boots, but the actor preferred his own plimsolls to give the Doctor a more casual air. The look was partly inspired by chef Jamie Oliver, who had appeared in a suit and sneakers for a March 19th episode of the chat show Parkinson on which Billie Piper was also a guest. With Eccleston having spoken in his natural Northern accent, it was felt that Tennant should not use his native Scottish brogue. Instead, the new Doctor would adopt a London accent, with dialogue -- later cut in editing -- suggesting that the new Doctor's speech patterns were based on Rose's.

As Season Twenty-Seven drew to a successful close, rumours began appearing in the press that Piper would be absent from some or all of the second run of episodes. These reports were finally quelled on June 15th, when the BBC confirmed that Piper would appear as Rose in both The Christmas Invasion and all of Season Twenty-Eight. What was not revealed at this point, however, 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. Meanwhile, on July 3rd, it was revealed that the Canadian Broadcasting Corporation would remain as a co-production partner for Tennant's first season.

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 episodes one and three, New Earth and School Reunion, to form Block One. The Christmas Invasion was generally referred to as “Episode X” to distinguish it from the regular run. Some consideration was given to moving to high-definition recording, but it was ultimately decided to retain the Digital Betacam format used during Season Twenty-Seven. During the four months which had elapsed since filming wrapped on the 2005 finale, Bad Wolf / The Parting Of The Ways, Piper had adopted a noticeably shorter hairstyle, not realising that the events of The Christmas Invasion would occur in its immediate aftermath. As a result, it was decided that she would wear hair extensions for the holiday special.

One of the actresses who auditioned for the role of Sally Jacobs was Freema Agyeman

Directing Block One was James Hawes, who had handled The Empty Child / The Doctor Dances the year before. One of the actresses he auditioned for the role of Sally Jacobs was Freema Agyeman. However, the production team felt that she was deserving of a more prominent role, and Hawes instead cast Icelandic actress Anita Briem, who had been in an episode of Doctors. It was decided that Agyeman would be kept in mind for a more suitable opportunity; after playing Adeola in the 2006 season finale, Army Of Ghosts / Doomsday, she would be seen as companion Martha Jones during Season Twenty-Nine.

On July 22nd, recording for The Christmas Invasion began in Newport, with the material which would appear on television screens during the special. First, Tredegar House was the setting for Harriet's appeal. Then, the dedicated Doctor Who studio space of Unit Q2 saw the Sycorax transmission taped against black drapes. A week in London followed, starting with the return of the TARDIS from the Sycorax ship at Wallis House in Brentford on July 25th. The main location used to represent the Powell Estate was the Brandon Estate in Kennington, as had been the case for Rose and Aliens Of London / World War Three a year earlier. Cameras rolled there on July 26th and 27th. Doctor Who then made history on the 28th, becoming the first television drama to be granted permission to film atop the Tower of London. The same day, plate shots were captured at various London landmarks for use in visual effects sequences. Cast and crew then returned to the Brandon Estate on July 29th, when they were visited by Peter Davison, who had played the Fifth Doctor. Both Davison and the Fourth Doctor, Tom Baker, had earlier sent well-wishes to Tennant as he embarked on his first Doctor Who story.

Torrential rain prevented some of the Powell Estate sequences from being completed in London. They were instead taped at Loudoun Square in Cardiff on August 1st. This meant that work on the TARDIS set at Unit Q2 was deferred to the 2nd. The same day, Hawes started filming material in the Tylers' flat at HTV Wales Studios in Cardiff; cast and crew remained there through the 5th. After the weekend, the team spent the first part of August 8th at Baltic House and Landmark Place in Cardiff, for shots of the blood-controlled masses ascending fire escapes. That night, the Christmas market could be found at The Hayes. The Cardiff city authorities had agreed to erect their yuletide decorations, while continuity with Rose was maintained by again dressing Howell's department store as Henrik's. A similar schedule was followed on August 9th, with the Sycorax victims emerging onto Broadstairs Road before Hawes' team returned to The Hayes.

For the interior of the Sycorax ship, Davies wanted to avoid a traditional metallic spacecraft. As such, these scenes were recorded at Clearwell Caves in Gloucestershire from August 10th to 13th. Davies hoped to reprise a shot originally intended for Rose, tracking from outside the TARDIS, through the doors and into the console room to reveal the revived Doctor. Unfortunately, these plans had to be abandoned due to the difficulty of transporting the motion-capture equipment into the caves. Indeed, the overall pace of the Gloucestershire work was very slow, hampered by both the technological limitations imposed by the environment and Sean Gilder's difficulties with the contact lenses he wore as the Sycorax Leader. This forced an additional day at Clearwell Caves on August 15th. Fortunately, Hawes was also able to complete the shots of Mickey in Clancy's Garage, at Brian Cox Motor Engineering in nearby Ellwood.

It was originally thought that the swordfight between the Doctor and the Sycorax Leader on the wing of the spaceship would be performed on the roof of a tall building in Cardiff. However, Hawes could not find a venue which obscured the surrounding conurbation, and so the sequence was instead filmed against an unobstructed view of the sea at the docks in Barry on August 16th. At the same time, a second unit used a green screen to capture shots of blood-controlled people standing on rooftops. August 17th, 18th and 19th were spent at Cardiff's Millennium Stadium, for scenes in the UNIT control centre. Davies' script specified that this was actually the headquarters of the British Rocket Group, an homage to Nigel Kneale's pioneering Quatermass serials of the Fifties.

August 22nd was again spent at HTV Wales Studios, for more material in the Tylers' flat and various shots against green screen. The shattering of the windows of the Powell Estate was filmed using miniatures at the BBC Model Unit Stage in Acton, London on September 6th and 7th. Inserts of the TARDIS console room were recorded at Unit Q2 on the 8th, while a shot of a Santa mask rolling on the ground was captured on the Unit Q2 car park on September 22nd.

Hogwarts-style robes in the TARDIS wardrobe alluded to David Tennant's involvement with Harry Potter and The Goblet Of Fire

Block Two was under way by the time Tennant filmed the TARDIS wardrobe scene at Unit Q2 on October 8th. The redressed console room set incorporated versions of the previous Doctors' outfits, including a multi-coloured scarf in the style of the Fourth Doctor's which had been knit for Collinson by his great-aunt. Further in-jokes included the appearance of Hogwarts-style robes, alluding to Tennant's involvement with the forthcoming feature film Harry Potter and The Goblet Of Fire, as well as a shirt in the style of Casanova. The same day, pick-up shots were also captured at Unit Q2 and on its parking lot, with similar work taking place at the studio on November 3rd. The last material recorded for The Christmas Invasion was the footage of the newsreaders, taped in the C2 News Studio at Cardiff's BBC Broadcasting House on November 10th.

Originally, Davies had hoped to emphasise the global threat of the Sycorax by including scenes of blood-controlled humans near the pyramids in Cairo, Egypt and on Sydney Harbour Bridge in Australia. However, neither shot could be realised convincingly, and they were both dropped. Also omitted in editing was the Tenth Doctor realising that his predecessor's “Fantastic!” catchphrase no longer suited him. Similarly, the concluding scene initially saw the Doctor and Rose ruminating on how they missed the Ninth Doctor, but the production team felt that the special should be looking to the future in its closing minutes, rather than reflecting on the past. Like Eccleston, it was planned that the closing credits would see Tennant billed as “Doctor Who”. At a late stage, however, the new lead actor asked to be credited as “the Doctor”, following the convention adopted throughout the Eighties.

On November 14th, Murray Gold recorded an extended version of his Doctor Who theme arrangement with the BBC National Orchestra of Wales. To accompany the scene of the Doctor in the TARDIS wardrobe, Davies originally wanted to use the 1963 version of The Bells of St Mary's by Bob B Soxx & the Blue Jeans. When the rights proved prohibitively expensive, Gold was instead asked to create an original tune. The result was Song For Ten, which adopted a similar Phil Spector style; Tim Phillips of the band CatHead provided the vocals.

As the broadcast of The Christmas Invasion approached, Doctor Who was given the rare distinction of appearing on the cover of the bumper holiday edition of the Radio Times. The specially-created artwork, depicting a snow globe with a TARDIS and a Dalek, marked the first time that a single programme had been spotlighted on the front of the Christmas issue since Carols From King's in 1989. The wave of publicity surrounding Doctor Who's first-ever Christmas special had the desired effect: The Christmas Invasion won the second-largest audience on December 25th, trailing EastEnders alone. And it also brought Doctor Who back into the Top Ten programmes for the week, which only Rose had previously accomplished. For the Doctor Who cast, crew and fans alike, it was a very merry Christmas indeed.

• Doctor Who Magazine Special Edition #14, 9th November 2006, “Episode X: The Christmas Invasion” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #51, 2015, “Story 167: The Christmas Invasion”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Inside Story by Gary Russell (2006), BBC Books.