Zeeta and Zelda Zamboni are beside themselves when they call the Monroe Detective Agency. "Our most prized possession has been stolen!" the sisters announce. "Will you help us? Please?" Max agrees to help them find their missing possession, and then he finds out what it is: a two-hundred-pound zucchini! Looking for a zucchini is "sure to be duller than dirt," Max thinks. However, once he and Uncle Larry (the ghost) start digging around for clues, they discover this new case is actually full of interesting possibilities. In fact, the Zamboni sisters have some awfully suspicious neighbors. The big question is, will Max and Uncle Larry manage to solve the mystery and locate the giant zucchini in time to enter it in the Harvest Fair's vegetable competition?