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—from old school rumors to a hidden trap door to . . . eyeballs! What secret treasures does the closet hold? Is it something more horrible than even Harry can imagine?