Horrible Harry has caused a lot of trouble at school, but he's never declared triple revenge on a classmate before! When Sid ruins an origami insect Song Lee made, Harry defends her -- by targeting Sid for some serious revenge. No one knows when, where, or how, but Harry is determined 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 horrible revenge mean Sid is the only one not invited to Harry's ninth birthday party?