The best Doctor Who episodes take a wholly unbelievable premise and work it into an enjoyable story. Steven Moffat proves himself more than up to the task, in Doctor Who’s “A Christmas Carol” with a relentless barrage of sharp dialogue and improbable scenarios that merge to form an unforgettable tangle of classic Dickens, classic Doctor Who, and a bit of J. J. Abrams.

Matt Smith seems well at ease as the chaotic Time Lord, even if mania is the new calm. The Doctor’s mania and cunning propel the story of a ship set to crash land on a planet ruled by one heartless, miserly old man. After being unable to reason with the Scrooge-like Kazran Sardick, (portrayed by Michael Gambon) Amy inadvertently provides the needed inspiration.

Does the key to Sardick’s heart lie in the frozen woman he describes as unimportant? Disbelievingly, The Doctor says “In 900 years of time and space I’ve never met anyone who wasn’t important before.”

The only slight failing of this Christmas special is the near-criminal underuse of Karen Gillan’s Amy Pond. She and her fiance spend the episode on the plummeting spaceship, being rocked about and viciously attacked by lens flares. Don’t be surprised if the shiny, shiny bridge of the distressed spaceliner reminds you of a recent cinematic reboot.

Of all the fanciful, outlandish oddities that present themselves in the episode, perhaps the most outlandish is reaching the limits of psychic paper: “It shorted out. Finally, a lie too big.” To learn that lie, and to visit a world of electric clouds and flying fish, you’ll have to watch it for yourself.