The kids in room 3B have found out their teacher is engaged! But who’s the lucky groom? Harry goes into detective mode to find out, and Doug is willing to help—until he sees Harry stealing a piece of evidence! Doug can’t believe what Harry has done, but Harry thinks his risky move is worth it. After all, he cracks the case and tells everyone who Miss Mackle is going to marry . . . but does Harry have his facts straight? Or has he made a horrible mistake?

Another uproarious caper in the Horrible Harry series, and this time Harry's not the one breaking the rules! Everyone in Room 3B knows that if Miss Mackle sees you playing with toys at your desk, she'll confiscate them for her June Box. So when a substitute teacher catches good girl Song Lee with a pink origami pig, into the June Box it goes. The problem is that this origami was to be a birthday present for Song Lee's grandmother. So Song Lee does the unthinkable: she breaks into the June Box to free her pig. Now Harry, Doug, and the rest of the kids have to work together--before they all wind up in horrible trouble!

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?

It’s a horrible day for Harry: a classmate is calling him names; his library book is overdue; he’s last in line for a turn to be the Room 3B VIP. Worst of all, it’s cold and icy outside again, which means no outdoor recess. It’s all too much for Harry—he starts to go cuckoo! Fortunately, there is a solution to Harry’s problems . . . but it comes from a very unexpected source.

Mary is always neat and organized, while Harry is horribly messy. But even Mary’s carefully planned schedule can’t help when things go wrong at her own birthday party. The party is supposed to start with the kids from Room 3B taking an exciting train ride, but when their train is delayed not one . . . not two . . . but three times, Mary’s plan is completely ruined! She thinks the only thing that can make her feel better is cup after cup of sugary Island Punch—even after her mom tells her to stop. But what Mary doesn’t know is that Harry has his own wacky ideas for saving Mary’s big day. Can the kids still have fun even when nothing’s going the way it’s supposed to, or will this be the most horrible birthday ever?

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.