Harry and his detective classmates try to find clues relating to a mysterious locked closet in Room 3B. Horrible Harry has discovered the biggest mystery ever in Room 3B! There's a locked closet in the classroom that's been sealed off for years, and he wants to find out what's inside. Harry can't solve this mystery alone, so Miss Mackle, Song Lee, and the rest of the class join in the detective work. Along the way they encounter some surprises --...
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.