It's December! Which means Tread Perilously becomes "Tread Merrily" -- but this year, it's all about A Christmas Carol and shows taking inspiration from the Charles Dickens classic. First up: Doctor Who's 2010 holiday special, "A Christmas Carol."

When Amy and Rory's honeymoon spaceship threatens to crash on a world of heavy clouds and Skysharks, The Doctor must convince the owner of the atmosphere control technology to help the ship land safely. Unfortunately, the owner, one Kazran Sardick, is quite the Scrooge. Luckily, a chance encounter with a recording of a Christmas carol inspires The Doctor to visit the bitter old man during his youth. Will generating a ghost of Christmas Past be enough for The Doctor to save his friends?

Erik tries to convince Justin "A Christmas Carol" is a good first episode for his daughter -- but it might really be "Dinosaurs on a Spaceship." Justin marvels at Karen Gillan's apparent agelessness. It leads to an appreciation of her work as Nebula. Erik manages to find a pretty good Quentyn Martell joke. His knowledge of Dickens' bibliography fails him at a key moment, however. Justin comes to believe every British actor hopes to yell at a poor family on stage. The Child Compressor makes another horrifying appearance and Justin re-imagines the Skysharks as a 1980s action figure line.