It's another rickety little mystery by the author of The Mystery on Crabapple Hill (1965) and again it features junior detectives who run into mystery as easily as most boys skin knees. Bunker and George find their suspect before they find out the crime--he's Mr. Hale, a neighbor at their summer lake resort, and he makes a project of studying the bats in a local cave. Some vague clues encourage the boys to go skindiving for a little black case which eventually turns out to contain part of a valuable coin collection. And they just barely miss giving away the whole show when they turn the goods over to the bogus game warden who is the real criminal. It's all genuine cliche.