Horrible Harry and the rest of Miss Mackle's class at South School guarantee fun for the holidays. It all starts when Miss Mackle's reading chair collapses while she's in it!
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.
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.
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.
The students in Miss Mickle's third-grade class enjoy a boat trip on the Connecticut River after a class discussion of ancestors reveals that Sidney and Ida both have connections to the sea.
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.
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.