When Sid, his annoying third-grade classmate, ruins Song Lee's origami praying mantis, Horrible Harry plots triple revenge on the troublemaker.

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?

Doug discovers that though being Harry's best friend in Miss Mackle's second grade class isn't always easy, as Harry likes to do horrible things, it is often a lot of fun.

Will the best artist win? The art teacher asks the students of Room 3B to draw posters of things they love, and one of them will get to display his or her artwork on South School's brand-new TV show. Harry doesn't care about the contest. Mary, on the other hand, is convinced that her poster is the best, and she can't wait for her television debut. But if Mary's poster doesn't get selected, then she's going to make horrible trouble for Room 3B--especially Harry!

Harry loves gym class-except for climbing rope. He's just too afraid of heights. But then a special valentine that Song Lee made for Harry goes missing. Now Harry must climb the rope in order to prove who took the valentine. But will he be able to overcome his fear? Or will Harry's Valentine's Day end horribly?

Third grade isn't looking too good for Herbie Jones. Not only is he stuck being in Apples, the lowest reading group in class, but he has to get a birthday present for annoying Annabelle and be good for the entire field trip to the museum! How is Herbie ever going to make it through the year?