Will wonders never cease? Chet Morton, the original coach potato, has joined Bayport's new state-of-the-art health club. But the Hardys know what's really on his mind, and her name is Dawn Reynolds, aerobics instructor. One mystery solved, an even bigger one awaits: The boys have found that working out in the club's weight room may be hazardous to their health.

Chet ends up in the hospital, and another of the club's members ends up in the bay...dead! Frank and Joe suspect that an underworld gang has muscled in on the action. The Hardys know they're in for the fight of their lives because when push comes to shove, these are the kind of thugs who'll stop pumping iron and start spraying lead!