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.

Horrible Harry, the class clown of Room 2B, returns to the spotlight with the Purple People, mysterious beings that only he can see.

Morty Hill worries that his best friend Molly Zander will get in trouble at school when a new classmate tells that Molly is the one who knocked off the principal's toupee.

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.

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.