Alvin thinks of himself as an expert on spooky stuff such as vampires and werewolves, so when a mysterious stranger moves in next door, Alvin's convinced he's a werewolf. Then, when Theodore gets bitten by a "dog", he starts chewing socks, eating meaty stuff, and is a lot faster, even Simon thinks something's not right. But at the moment, he's more like a werepuppy. They now have to find a way to cure him before the next full moon – during the school play, "Dr. Jekyll and Mr. Hyde" – or else Theodore will become a werewolf – forever!
