Plus, the Scrooge character is played by Michael Gambon, the Dumbledore of the later Harry Potter movies, and his younger self who also features heavily in the episode is Laurence Belcher, probably best known as the young Xavier in X-Men: First Class. Really, quite a gifted kid. And while Amy + Rory feature on the edges, this is really Matt Smith's story. Imagine Matt Smith and Michael Gambon engaged in an existential battle for an old man's soul, throw in a heap of Chrstmas spirit and faerie, and you've got the idea.
It's only $2 at Amazon, or free if you have Amazon Prime. Do check it out. I suspect it's probably free through Netflix as well, but I don't have that service, so I can't offer a link. (If anyone does find it there let me know and I'll post a link here.) I liked it!