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.
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.
The students in Miss Mickle's third-grade class enjoy a boat trip on the Connecticut River after a class discussion of ancestors reveal that Sidney and Ida both have connections to the sea.
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.
Third-grader Harry thinks of a way to patch up a conflict with his schoolmate Mary and include his friends in a fun detective case with rewards for everyone, including Grandpa.
When the origami pig that Song Lee made for her grandmother is seized by a substitute teacher, the usually well-behaved third-grader breaks a classroom rule to rescue it from the "June box."
"The kids in Room 3B are asked to draw posters of things that they love, and one of them will get to share his or her poster on TV"--Provided by publisher.
Selected to join the school safety patrol squad, third-grader Mary quickly turns into a bully and classmate Doug suspects she may be tampering with the book raffle.
"When his best friend Doug discovers his top-secret hideout, Harry enlists his friends' help to save it from being developed into an apartment building"--
When his friend Doug is infested with head lice and Mary convinces everyone to stay away, Harry and his other friend Song Lee stand by Doug and declare war on the bugs.
Spurred on by the promise of a mystery prize, Harry and Doug are determined to win the most points on Field Day, but Harry's sneaky tactics annoy their competition, making the two boys targets for revenge.
As the last day of third grade looms, Doug learns that Harry is moving to a new town and wonders how he'll survive fourth grade without him, while Harry wonders about the new neighbors across the street and what they could mean for their friendship.
