Amy and Rory are on their honeymoon during the episode, and run into trouble when the pleasure cruise spaceship they're travelling on finds itself under attack from "something very nasty". The spaceship is orbiting an alien planet, which is where the Doctor is for a considerate part of the story. This therefore causes Amy and Rory to be separated from the Time Lord for the majority of the episode.
Showrunner Steven Moffat has said 'Oh, we're going for broke with this one. It's all your favorite Christmas movies at once, in an hour, with monsters. And the Doctor. And a honeymoon. And ... oh, you'll see. I've honestly never been so excited about writing anything!'
The special which features special stars Michael Gambon and Katherine Jenkins, will air Christmas day on BBC one and in the US on BBC America too.
Also check out the Matt Smith being interviewed on US talk show 'The
Late Late Show With Craig Ferguson' in the player below: