When the diamond on Miss Mackle’s engagement ring disappears, the kids in Room 3B are determined to help her find it—especially Harry. The third grade talent show is coming up, and he wants to show off his skills as South School’s greatest detective. Meanwhile, Doug has no idea what to do for his act. He can’t sing like Dexter, or dance like Ida, or solve mysteries like Harry. Now the best friends have to find both Doug’s talent and the teacher’s diamond, or else the talent show will be a horrible disaster for everyone!

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?

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.

A cold snap has everyone in the town of Toby Mills feeling down, until the mayor's wife thinks of a way to warm things up again.

Horrible Harry says goodbye in the 30th anniversary finale of a beloved series! The last day of third grade is approaching fast, but a farewell to Room 3B isn't the only goodbye Doug has to say this year. Song Lee and Mary are busy preparing an end-of-year present for their favorite teacher, Mrs. Flaubert, but their excitement can't distract Doug from Harry's suspicious silence. When Harry finally admits that his family is moving to a whole other town, Doug can't believe his ears. How will he survive fourth grade without his best friend?! But Harry has a more pressing question: who's that moving in to the empty house across the street from him? And what could these new neighbors mean for their friendship?