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.

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.

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.

Horrible Harry and friends start off third grade with a new room and a field trip to an old copper mine to study rocks.

Harry and Doug investigate when special bookmarks begin to disappear while their class is busy studying poetry and running their own post office.

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!

"It's Mary's birthday party. When things go horribly wrong will Harry be able to save the day?"--

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."

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.

Harry and his detective classmates try to find clues relating to a mysterious locked closet in Room 3B.