Doctor: Peter Capaldi (Twelfth Doctor)
Companion: Jenna Coleman (Clara Oswald)
Nick Frost – Santa Claus
Samuel Anderson – Danny Pink
Dan Starkey – Ian
Nathan McMullen – Wolf
Faye Marsay – Shona McCullough
Natalie Gumede – Ashley Carter
Maureen Beattie – Fiona Bellows
Michael Troughton – Professor Albert Smithe
Directed by Paul Wilmshurst
Written by Steven Moffat
Script editor David P Davis
Produced by Paul Frift
Incidental music composer Murray Gold
Length 60 minutes
Originally broadcast 25 December 2014
On Christmas Eve, Clara Oswald is awakened by the sound of something crashing on her rooftop. Investigating nearby voices, she is astonished to find Santa Claus, two elves, a sleigh and flying reindeer stranded on her roof. The Doctor soon arrives to take Clara away, but not before Santa tells him that he will need his help before the night is over, while promising him a tangerine.
The crab-like creatures (also referred to as Facehuggers in the episode), shown here at the Doctor Who Symphonic Spectacular.
At the North Pole, a group of scientists attempt to save their fellow base personnel, who have been taken over by crab-like creatures. Their effort is interrupted as the Doctor and Clara arrive on the scene. While introductions are being made, however, they come under attack from the crabs, only to be rescued by Santa, who also provides a specimen of one of the creatures. The Doctor identifies the creatures as Dream Crabs – aliens that induce pleasant, euphoric dreams on their intended victims as a distraction whilst the crabs devour their brains. During this time, both the Doctor and Clara reveal that they had lied to each other during their last meeting: Clara lied about her dead boyfriend, Danny Pink, having returned from the Nethersphere, while the Doctor falsely claimed to have found Gallifrey. While investigating the Dream Crabs, the Doctor sends Clara to recover Santa's specimen, only for him to remember that they use telepathic connections to their intended victims to see their surroundings, but only when they are being thought about. This causes the specimen in the lab to come alive and attack Clara.
Sent into a dream world, Clara is reunited with Danny, but also discovers chalkboards with messages (symbolising the Doctor's attempts to communicate with her) warning her that she is, in fact, dying. However, Clara deliberately ignores them, continuing to focus on Danny. The Doctor willingly falls victim to another Dream Crab in order to enter her dream world and confront her. She still resists willing herself to wake up, until Danny – already aware of his and Clara's dream state – tells her that while she can miss him, she must also move on with her life. As Clara accepts the reality of his death, she and the Doctor both wake up, discovering that the creatures die and turn into dust if removed from a host prematurely.
After waking up back in the research station, however, the Doctor deduces that they haven't truly woken up – they are simply in a different layer of a multi-faceted dream and have been since the initial attack when they arrived. In reality, none of them actually escaped it. The Doctor proves this by having the scientists individually read copies of the base's manual that each had been issued. They find that the first word of any chosen page is different for each of them, rather than identical as it should be. The Doctor further explains that the Dream Crabs could not stop their subconscious minds from fighting back, which manifested in the form of Santa Claus. Santa himself clarifies the entire situation to the group, and he aids them in waking up.
As the Doctor prepares to abandon the base, Clara reminds him that they met Santa before arriving, proving that the entire scenario thus far has also been a dream. The reality is, all of the scientists are strangers from different times and places whose dreams are linked with the Doctor and Clara through the Dream Crabs. One of the scientists soon falls victim to the infected personnel, revealed to be manifestations of the "crew's" minds that have already surrendered to their fate. Infected visages of the Doctor and Clara also emerge from the TARDIS, and the group find themselves surrounded. Facing defeat, the Doctor reminds everyone that Santa helped them survive the previous attack and has them dream of Santa, who arrives just in time on his sleigh and flies them to safety. The remaining "scientists" soon realize that they are, in fact, ordinary people: a wheelchair-bound grandmother, an account manager for a perfumer, and a lonely shop girl. They and the Doctor soon wake up to their real lives, one by one, until only Clara is left.
Upon waking, the Doctor traces the psychic signal linking their dreams back to Clara and is able to pull the crab off, only to learn that Clara is now an elderly woman. Sixty-two years have apparently elapsed since they last met. As they become reacquainted, the Doctor expresses regret over not coming back for her sooner. Suddenly, Santa appears and encourages the Doctor to act before it is too late. The Doctor, realizing he is still dreaming, wakes up for real and rushes to the real Clara. He frees her from the crab to find, to their mutual relief, that she is still a young adult. Wanting no regrets and offering her all of time and space, the Doctor implores Clara to join him again. Clara happily accepts. As the pair depart, they are unaware of a tangerine having been left on Clara's windowsill.