Frank and Joe go undercover as big-city bicycle messengers to crack a gang of high-tech thieves.

The Hardys are sailing a friend's yacht to the coast of Florida when a 5,000-ton cargo ship appears out of nowhere and nearly sends the yacht to the bottom. When the boys investigate they discover that the ship's crew is missing.

Caught in their motorboat the "Sleuth", by a sudden storm at sea, Frank and Joe are helpless when the engine conks out. Drifting farther from shore amid the tumultuous waves, the boys are relieved to see the lights of a yacht. But their SOS is ignored.

Frank and Joe are on the fast track to danger when they try to find out who stole a multi-million dollar collection of model trains.

Frank and Joe Hardy are headed for a high-seas showdown when their trip to the Everglades for the Swampland Rodeo lands them in the middle of a Miami crime spree.

Frank and Joe Hardy are drawn into a tangled web of danger when they are called in to investigate mysterious accidents plaguing a college football star.

The Hardy brothers travel to an Arizona ranch and eventually, Thailand in pursuit of jewel thieves who have stolen a priceless ruby.

On a visit to Germany the Hardy brothers investigate the theft of plans for a newly invented submarine and the mysterious disappearance of valuable coins and paintings.

The Hardy boys, vacationing in Vermont, offer to help the police locate a missing camper and find themselves involved with a doomsday cult, a deadly strain of bacteria, and possibly the devil.