While riding their motorcycles along the Shore Road, Frank and Joe Hardy, along with their pal Chet Morton, meet a local fisherman who has just had his car stolen. The three boys give chase and, further down the road, find their chum, Jack Dodd, and his father being questioned by the police because a stolen car was found in their yard. The Hardy boys, of course, resolve to solve the case and clear their friend's name, as many other auto thefts have also occurred recently. They soon discover a band of tramps camping on the beach, a man named Gus Montrose who used to work for Mr. Dodd, and a group of fishermen sneaking around the woods at night. Although threatened by Dodd, thwarted at every turn to unravel the mystery of the car thefts, and using their own money to purchase and hide in the trunk of a "bait car," the boys enter the thieves' lair, encounter their armed adversaries, and solve the Shore Road Mystery!