It's Christmas on the Riverbank, and Hammy and Martha and GP and Turtle set out to find a tree and decorate it and make presents for each other. But a mysterious stranger steals the tree and they find it back in the woods near a cave. A monstrous voice and a shadow on the cave wall frighten them off, but they find the monster is a lonely little Gerbil called Humbug, and they win him over by sharing their Christmas with him.