The kids in room 3B have found out that their teacher, Miss Mackle, is engaged! But who is the lucky groom? Leave it to Harry and his best friend, Doug, to find out. Harry goes into detective mode, and wastes no time starting his investigation.

Doug is willing to help—until he sees Harry stealing a piece of evidence. Doug can't believe it! But Harry thinks his risky move is worth it.

After all, he manages to crack the case and tells everyone who Miss Mackle is going to marry. But does Harry really have all the facts, or has he made a horrible mistake?