The first scenes of the annual Dr Who Christmas Special introduce the viewer to Nick Frost's guest turn as Santa Claus, complete with sledge, reindeer and two lippy elves but from there the story gets progressively darker, exploring the darker psyche of humanity as the Doctor, pleasingly reunited with Clara, lands at an Arctic scientific outpost where along with its four inhabitants, they find themselves in a waking dream scenario which threatens to kill them all.
Writer Steven Moffat nods at various sci-fi / horror classic films of yesteryear, notably "Alien", "Nightmare on Elm Street", "Inception" even "Ghostbusters" as well as Christmas films "A Christmas Carol" and "The Santa Clause" and weaves a densely layered tale which takes a bit of following, but like Santa on Christmas Eve, gets there in the end.
As one would imagine Santa Frost and his two impish assistants get the majority of the funny lines (when asked how he can deliver so many Christmas presents from a single sled, he knowingly counters "Bigger on the inside" and otherwise becomes the seasonal focus of the group's thought processes to thwart the Dream Crab's life-threatening menace. There were some neat touches in the story like Clara's death-prophesying blackboard and the way the Doctor uses random book-reading to alert them to their danger although some other devices were unnecessary in my opinion, most notably the "return" of Danny Pink into Clara's life and the once too often used collective resistance to break the monsters' spell.
With barely any day-light shots and being even more set-bound than usual, this Dr Who tale effectively builds up its claustrophobic atmosphere of life-threatening dread until the bearded weirdie in red comes along to save the day. There's an interesting diversion near the end as the Doctor visits a much older Clara from 60 years in the future to play out a blame-and-redemption tete-a-tete before he saves the (Christmas) day and hooks up again with Clara in the TARDIS, just in time for the next series.
This wasn't the best Dr Who Christmas special I've seen but had its moments and definitely deserved its prime-time slot on the Christmas schedules.