After four days in a row of indoor recess, Miss Mackle's students are bored—until Harry decides to solve 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 know what's inside. So Harry drafts Doug, Song Lee, and the rest of the class to help him investigate.

Their search reveals many surprises—from a hidden trapdoor to... eyeballs! What secret treasures does the closet hold? Is it something more horrible than even Harry can imagine?