Horrible Harry has caused a lot of trouble at school, but this time, he's taken it to a new level. 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 back at Sid! Will it be on Pajama Day, when the kids wear their PJs to school? Will it happen in Room 3B? Or will Harry's horrible plan mean Sid is the only one not invited to Harry's ninth birthday party?