Tod's ears are so clogged up he can't hear, but he doesn't want to miss school. His teacher, the ex-spy Mr. Merlin, is sure to have secret codes for the class to figure out and a new mystery for them to solve. but the mystery turns out to be in Todd's own backyard. Someone's been in his tree house and left a pretty yucky clue. It's another case for Mr. Merlin's police scientist friend, Dr. Smiley! You can help decode the clues and solve the mysteries in these other stories about the Third-Grade Detective.