A space liner carrying 4003 passengers, including Amy Pond (Karen Gillan) and Rory Williams (Arthur Darvill) on their honeymoon, loses control when passing through strange electrified clouds over a human-inhabited planet. The Doctor (Matt Smith), summoned by Amy, is unable to use the TARDIS to save the ship, and instead lands on the planet and discovers that a spire in the centre of a large city is influencing the atmosphere. Its owner, the bitter and peevish old Kazran Sardick (Michael Gambon), refuses to operate the "isomorphic" controls to deactivate the spire and allow the ship to land safely. The Doctor observes Kazran's fear of his father who had built the spire, and creates a scheme inspired by Charles Dickens' A Christmas Carol to make Kazran kinder.
He ventures to Kazran's past and meets him as a kind-hearted young boy, interested in the unique properties of the planet's atmosphere that allow fish to swim in it. The Doctor experiments with Kazran, leading to a shark entering the room and swallowing the Doctor's sonic screwdriver. Though the Doctor is able to recover part of the screwdriver, the shark is wounded in the effort, unable to swim back into the atmosphere. Kazran offers a solution by taking him to a cryogenic storeroom where his father has kept people in storage as "security" for loans, including Abigail (Katherine Jenkins), a young woman with whom Kazran has been enamored. Abigail, once released, sings to soothe the shark while the Doctor uses Abigail's cryo-unit to transport the shark back to the atmosphere. As the Doctor and Kazran return Abigail to storage, Kazran promises her they will see her every Christmas Eve. The Doctor keeps this promise, using the TARDIS to jump forward each year, helping to reunite Kazran and Abigail and watching their relationship blossom with the two falling in love and eventually kissing. However, after one such visit, Abigail tells the now young adult Kazran a secret, and Kazran requests the Doctor end the practice, keeping Abigail in storage indefinitely. Though old Kazran in the present becomes pleased with his new memories, he remains bitter at Abigail's fate and refuses to help save the ship. As a result of the Doctor's actions, instead of being cruel and heartless, Kazran is bitter and heartbroken.
Old Kazran is soon visited by holographic images of the ship's crew in the present. After observing the effects of Abigail's singing, the crew is singing Christmas carols, the sound frequencies of which are helping to stabilise the ship but cannot prevent it from crashing. Amy appears to Kazran and implores his help, but he waves away the holograms. The Doctor appears, and Kazran surmises that he is there to show Kazran his future, but he could not care less if he dies old and alone; he reveals that Abigail had an incurable disease on entering cryostorage and will only have one more day to live. As he cannot decide which day that should be, Kazran chastises the Doctor for believing he could change his mind. Unbeknownst to Kazran, the Doctor has brought young Kazran with him; the realisation of how much his bitter future self now resembles his feared father causes a change of heart in Kazran (making his own memory of his future self the show's version of the ghost of Christmas yet to come), and he quickly agrees to release the controls to save the ship.
The Doctor finds his changes to Kazran's past have locked him out from the spire's controls, but the Doctor devises a solution: by having Abigail sing through one half of the broken sonic screwdriver, the other half, still in the shark, will resonate in the atmosphere and disrupt the storm to allow the ship to land safely. Kazran releases Abigail knowing this will be the last time, but Abigail understands and believes it is time for them to share a Christmas Day. The plan works successfully, and the resulting cloud break-out creates snowfall that falls around the city. As the Doctor rejoins Amy and Rory and prepares to take young Kazran back to the past, old Kazran and Abigail enjoy one last shark-led carriage-ride together.
Several nods to earlier outfits in the series appear in "A Christmas Carol". Amy Pond wears her kissogram policewoman's outfit from "The Eleventh Hour", while Rory wears a Roman centurion's outfit as seen in "The Pandorica Opens". In one of the many Christmas Eves the Doctor and Kazran spend with Abigail, they present themselves to her in long, stripy scarves, the Fourth Doctor's trademark accessory. The two also appear in fezzes, an item of clothing the Doctor became fond of in "The Big Bang". Kazran's controls are "isomorphic", which means that only he can use them. In Pyramids of Mars (1975), the TARDIS's controls are said to be isomorphic, though in subsequent adventures this feature was retained or ignored as the plot demands. in last of the time lords (2007) The Master's Laser Screwdriver is "isomorphic".
Writer Steven Moffat, who is also head writer and executive producer of the series, wanted to make the special "really Christmassy" because the previous Christmas special, The End of Time, had been darker than usual as it led to the Tenth Doctor's (David Tennant) regeneration. He stated that he had "never been so excited about writing anything. I was laughing madly as I typed along to Christmas songs in April". Moffat stated that A Christmas Carol was "probably [his] favourite Christmas story" and that it lent itself to Doctor Who, as there was an aspect similar to time travel in the story. He also pointed out that the Doctor intentionally based his reform of Kazran on Dickens' story; Dickens exists as a character in the Doctor Who universe, having appeared in the 2005 episode "The Unquiet Dead". The concept of the sky shark was based on Moffat's childhood fear of sharks which had evolved to swim outside the water.
Moffat noted that Kazran was unlike other villains found in Doctor Who, as he was not completely "wicked". Instead he was more of a "damaged" character; the Doctor recognises this when Kazran demonstrates his inability to hit a little boy, due to it reminding him of when his father beat him. This allowed the Doctor to want to change his past and "defrost" his soul. The phrase "halfway out of the dark" is used in the episode, a reference to the fact that the hard winter is almost over as well as a metaphor for Kazran.
Starting with this episode, Arthur Darvill achieved regular status. British actor Michael Gambon was cast in the role of Kazran. Andy Pryor, the casting director, did not believe Gambon would be available, and was surprised when Gambon accepted the role. Smith, Gillan, and Darvill were very honoured to work with him. Moffat stated, "Michael Gambon is as distinguished an actor as I can imagine and the fact that he was Dumbledore means that he is already known to millions of children". Kazran was played as a boy by Laurence Belcher and as a young adult by Danny Horn. Director Toby Haynes said that acting came naturally to Belcher and he "captured the story" and drew the audience in.
The episode also features the acting debut of Welsh singer Katherine Jenkins. Shortly before her 30th birthday in June 2010, Jenkins was asked about the part, and she accepted. Moffat was unaware that Jenkins had no former acting experience. Jenkins originally was not interested in acting, but thought that she would "like to try" Doctor Who, as it was "such an iconic show". Producer Sanne Wohlenberg believed that Jenkins was "a perfect fit" for the role. Though she was nervous, Jenkins found the Doctor Who team "encouraging and supportive" and believed she would act again "if the right thing came along".
A read-through took place in Cardiff on Thursday, 8 July and production started on 12 July 2010 and lasted into August 2010. The episode marked the debut of Michael Pickwood as the new set designer. Pickwood and Haynes worked together to create the town, incorporating elements that would be needed in a society that shared its daily life with fish. Nearly all windows in the town are round, reminiscent of submarines. The structures were made out of metal, filmed at a local steel works. Kazran's study was designed to be very large, as Kazran spent several scenes in there alone and would feel "dwarfed" by it. The walls were intended to look like copper that had turned green, which when combined with the red drapes created the Christmas colours of red and green. The painting of Kazran's father was created by taking a picture of Gambon, lightly printing it on a canvas, and painting in the details. Kazran's controls were inspired by a church organ.
The Doctor's first scene in the episode is falling down the chimney and presenting himself to Kazran. Smith was pleased with the entrance of his character, noting similarities between the Doctor and Santa Claus. However, a stuntman performed the scene. For the scene in which a sky fish nibbles on the sonic screwdriver, the screwdriver was dangled in front of a greenscreen and tapped with a pen to create the effect of the fish nudging it. To maintain secrecy of the episode's plot, the model shark was given the codename "Clive". The scenes in which the characters ride through the air in a sleigh pulled by one of the flying sharks was filmed in front of a greenscreen. The sleigh was in fact a rickshaw which crew members rocked back and forth as a wind machine created the effect of flying through the air. As it was impossible to film underneath the rickshaw, a miniature model of the sleigh was used for the shots underneath the sleigh rather than computer-generated effects.
The episode features a song specifically written for Jenkins, titled "Abigail's Song". In the script it was written that the song was unique and specific to Doctor Who, and composer Murray Gold was contacted to write the song. Jenkins did a demo and then sang the song while filming the actual scene, as she thought it would look more natural. Afterwards she sang the final version and new, lighter arrangement was composed by the National Orchestra of Wales. Selected pieces of music from this special, as composed by Gold, were released on a soundtrack 21 March 2011 by Silva Screen Records.
"A Christmas Carol" was first broadcast in the United Kingdom on BBC One on Christmas Day 2010. According to overnight figures, "A Christmas Carol" was tied with Come Fly with Me as the second most-watched programme on Christmas Day in the United Kingdom, behind EastEnders, and with an average viewership of 10.3 million peaking at 10.7 million. The final rating for BBC One was 12.11, making the episode the fourth highest rated of the entire canon since the series was revived in 2005, behind previous specials "Voyage of the Damned" (13.31 million, 2007), "The Next Doctor" (13.10 million, 2008), and Part Two of The End of Time (12.27 million, 2010). "A Christmas Carol" was the third highest rated show on all UK TV for the week ending 26 December 2010. In addition, 716,000 watched the show on the BBC's iPlayer, making it the eighth most downloaded show for December 2010. This special had an Appreciation Index of 83.
"A Christmas Carol" is the first episode of Doctor Who that was broadcast the same day in the United Kingdom and in the United States on BBC America. In the United States, 727,000 viewers watched "A Christmas Carol", an 8% increase on the previous holiday special, part one of The End of Time. The special was broadcast in Canada on Space on 26 December and in Australia on ABC1 on the same day, where it had overnight ratings of 880,000 viewers.
The episode received generally positive reviews. Dan Martin of The Guardian described the episode as a "sumptuous triumph". Website Den of Geek's Simon Brew applauded Moffat for "not taking the easy way out" with the adaptation, saying it was "really quite mad, undoubtedly festive, and it treats the Dickens source material with respect". However, he also wondered if young children "would have enjoyed 'A Christmas Carol' quite as much" as adults". Keith Phipps of The A.V. Club gave the special an A-, naming it the "best Doctor Who Christmas special" he had seen. He praised Gambon, Smith, both actors playing the younger Kazran and thought Gillan and Darvill "[made] the most of what they're given", though he commented that "Katherine Jenkins is more an ethereal presence with a lovely voice...than an actress".
Dave Golder of SFX gave the episode four and a half out of five stars, explaining that, although there were "creaky" moments, it was "the most adult Christmas special we've yet been given, with some complex story-telling techniques, a plot driven by the characters and some quite mind-bending concepts". He praised Jenkins' debut and Smith, whom he described as "a force of nature unleashed on screen". IGN's Cindy White rated the episode 8 out of 10, describing it as a "clever remix" of A Christmas Carol and praising Smith, Gambon, and Jenkins. Brad Trechak, writing for TV Squad, praised Moffat's "expertly woven story". MTV's Rick Marshall called it "easily one of the best episodes of the series' modern era...populated with just the right amount of humor, drama, scares, and sentimental reverence for the classic story that inspired its narrative".
Sam McPherson of Zap2it said that he "enjoyed 'A Christmas Carol' more than I have almost any other special that has aired since the series restarted in 2005", but thought that "some parts were absolutely silly, and others were absolutely incoherent" due to the frantic pace. While he considered Amy and Rory "criminally underused", they were responsible for "heavy laughs" and left the Doctor to be the "delight of the episode". The Daily Telegraph's Chris Harvey was less enthusiastic about the episode, saying that it "started nicely", but that "by the time she was singing to the shark, I'd had enough," while thinking that, "It's not really for old curmudgeons like me, who got more of a kick out of Michael Gambon's miserly Kazran Sardick when he was sneering and snarling at the beginning of the episode than when he had been thoroughly heartwarmed by the end."
"A Christmas Carol" was nominated for the 2011 Hugo Award for Best Dramatic Presentation (Short Form), but lost to the preceding series finale "The Pandorica Opens"/"The Big Bang".
"A Christmas Carol" was released on DVD and Blu-ray as a standalone in Region 2 on 24 January 2011, followed by Region 1 release in the two formats on 15 February 2011. It contained the associated Doctor Who Confidential for the episode, as well as a cut-down version of the Doctor Who Prom. The special was also included in the Complete Sixth Series box set released on 21 November 2011 (Region 2) and 22 November (Region 1).
The ten Christmas specials between "The Christmas Invasion" and "Last Christmas" inclusive were released in a boxset titled Doctor Who – The 10 Christmas Specials on 19 October 2015.