On the last day of second grade, Song Lee's party at the local amusement park forces Harry to face his fear of elevators when he is expected to ride the Drop of Doom.
When Horrible Harry's pet cat, Googer, sneaks a ride and gets into the school building, third-graders Harry and Doug have to catch him before anyone else does, which means going into some forbidden places.
Harry's interest in solving mysteries has caught the attention of his classmates in 3B, so he challenges them to discover what he has hidden in a giant, old suitcase.
Miss Mackle's third graders share December holiday traditions--Christmas, Hanukkah, Kwanzaa, Three Kings Day, and Korean New Year--with each other and with Harry's great grandfather, who just moved to a nursing home.