Thirteen-year-old Dexter Hart is afraid that he will be bored at his family's vacation spot on a lake in Vermont-- a perceptive precognition judging from the feeble mystery that finally grabs his interest. Dexter's singleminded enthusiasm over anything mechanical is shared by his tomboyish neighbor Ab (Abigail). Martin who becomes his collaborator. The problem involves some petty thievery and a hundred dollar reward has been offered for the culprit. The youngsters finally clue in through some deep-lake diving, and catch some spoiled scions from the wealthy side of town... It seems like a lot of overly energetic exercise to fit the crime or the reward.