On Christmas Eve, Clara Oswald (Jenna Coleman) finds Santa Claus (Nick Frost) stranded on her roof. The Doctor (Peter Capaldi) shows up to take Clara away and Santa says the Doctor will need his help later. The Doctor and Clara travel to a military base at the North Pole, where four of the crew are in the infirmary being devoured by dream crabs. These are crablike aliens that induce dreams in their victims while devouring their brains. Nice.

A group of scientists is trying to understand these aliens, which are telepathic and can observe the surroundings of anyone who thinks about a crab. So, try not to think about a crab. The Doctor, Clara, and the rest of the crew are attacked by more crabs, but Santa rescues them. After confessing that each has lied to the other, the Doctor sends Clara to recover Santa’s crab specimen. But she thinks about crabs, and it comes alive and attacks her.

In a dream, she reunites with Danny, her dead lover. To enter her dream and save her, the Doctor allows a crab to attack him. Danny tells her to move on with her life and she and the Doctor wake up. Waking up, it appears, kills the crab. The Doctor suspects, however, that they are still in a dream and Santa is a manifestation of the subconscious trying to fight back. Santa wakes them all up.

Clara reminds the Doctor that they saw Santa before the dream began and she thinks none of the scientists are real. The Doctor, Clara, and the scientists dream of Santa, and he wakes them up to their real lives, taking them home in his sleigh. One by one, they awaken, and the crabs die. But Clara is now an elderly woman, still with a crab in her mind. The Doctor wakes her up and she reverts to being a young woman. They leave in the TARDIS.

The episode received critical acclaim as both fun and scary. Someone said it was Inception meets Alien. There are references to Alien by Ridley Scott, and coincidentally, Scott was a trainee set-designer for Doctor Who in 1963. He was supposed to design the Daleks, but because of a scheduling conflict was replaced by Raymond Cusick. Dan Starkey (Strax) appears as Ian the Elf without Sontaran makeup. It was the third Christmas story for Starkey, and for Jenna Coleman, who was the only companion thus honoured. Michael Troughton, son of Second Doctor Patrick Troughton, also appears.

A character named Shona was played by Faye Marsay, who played The Waif in Game of Thrones. Originally, Clara was supposed to die of old age so she could leave the role, but Peter Capaldi and Steven Moffat asked her not to leave and she decided to stay, and the end was quickly rewritten. When the sceptical Doctor asks Santa how he fits all those toys in one bag, he says it’s bigger on the inside.