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.
Follows the school activities of second-graders, Harry and Doug, as they participate in a secret mission to celebrate "Charlotte's Web," learn how to make green slime, and put on a skit warning about the dangers of smoking.
It's almost summer vacation and the students in room 2B are having trouble concentrating on their butterfly project, until a frightening-looking new teacher comes to supervise the Suspension Room and Harry is sent there as punishment for unruly behavior.
Harry's huge imagination gets him in trouble when he tries to convince everyone that his mysterous Purple People will be coming to the classes' tea party.
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 Sidney kills Harry's pet spider, Harry wants revenge. And what better place to get it than on the class trip to a creepy old prison and copper mine.
Room 3B just learned that Ida had an ancestor who sailed on the Titanic and lived to tell about it! Miss Mackle decides that she will take the class on a real riverboat trip. But has something gone wrong.
Horrible Harry and Song Lee have been best friends since kindergarten. But when they have to work together on a project about dragons, they get into a fight that threatens to end their friendship.
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 has just moved to a nursing home.
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.