Doctor Who: Last Christmas (2014)
The Jolly Old Elf is played by Nick Frost, who might be the single best named actor to ever take on the role. He does a fantastic job with it, too, embracing the absurdity of a fantasy character showing up on a science fiction show.
The opening focuses on Clara, still grieving, rushing to her roof to investigate a loud crash. She discovers Santa Claus and two elves trying to corral their flying reindeer. The TARDIS shows up in the middle of this, and The Doctor appears to trade barbs with Santa before taking Clara away. They soon arrive at the North Pole, where they discover a group of scientists dealing with an alien outbreak.
The aliens, called Dream Crabs, are drawn to people who are thinking about them. They then wrap around their heads and put their host in a catatonic state, feeding their victims dreams to keep them docile while they dissolve and devour their brains.
This is highly reminiscent of the facehuggers from the Alien movies, a fact one of the scientists notes. As the episode progresses, the Doctor slowly realizes that everything they're seeing is a dream. Or, more accurately, a dream within a dream within... you get the idea. That's also the stated rationale behind having Santa present: he's the subconscious manifestation of their minds fighting back. Or something.
Things get even weirder as they slowly Inception their way back towards reality. Rather than use this as a one-off, the writers addressed the darker turns from the previous series. The Doctor learns that Clara's fiance didn't survive, and Clara learns the Doctor didn't find his home.
The episode ends with a sleigh ride with Santa, which allows three of the four "scientists" to wake up. Shockingly, the one who doesn't make it is the sole male crew member, which is essentially unheard of for this show. The Christmas episodes, in particular, usually seem to have it out for the female guest star. I guess someone finally noticed the pattern.
Clara and the Doctor have one last "dream within a dream" to go through, in which they realize they need each other. Once all that's settled, they take off in the TARDIS before the camera pulls back and implies either that Santa actually is real or they're still trapped within a dream - take your pick.
Because there's so much going on, and the tone shifts so much, the episode is tough to rate. The comedic bits with Santa and his elves are hilarious, while the Dream Crabs are among the creepiest monsters the show's portrayed. When this delves into horror, it gets scary. The Inception stuff, however, oscillates between fun and cheesy, with the latter overused around Clara's loss. Her fiance's death at the end of the last season was undercut by the lackluster story line around it. As a result, it's difficult to take any of this seriously: too much is built on melodrama.
But it's still a good episode over all. Not one of the best, but Frost's Santa Claus is more than enough reason to watch it, provided you have some background with the show (this one's a little too embedded in continuity to recommend as a jumping on point).