The Hardy brothers, Frank and Joe, are trying to find out who is behind a rash of automobile thefts occurring on Shore Road just outside of Bayport.

It becomes much more serious when one of the boys' friends, Jack Dodd and his father are accused of the thefts. Initially the young detectives had been helping the Dodds track down a long lost family treasure from the time of the Pilgrims. And then cars started to disappear.