Horrible Harry has caused a lot of trouble at school before, but he's never caused trouble like this. When Sid ruins an origami animal Song Lee made, Harry defends her by declaring triple revenge! No one knows when, where, or how, but Harry is going to get Sid back. Will it be on pajama day, when the kids wear their nightclothes to school? Will it happen in Room 3B? Or will Harry's ultimate revenge mean Sid is the only one not invited to Harry's ninth birthday party?