A sleek new hydrofoil is scheduled to start ferrying passengers between Bayport and Cape Cutlass. But business enemies of the hydrofoil owner have stirred up a hornet's nest of violent opposition among small boat owners. Fearing sabotage, he begs Frank and Joe Hardy to guard the Flying Express on her maiden trip.

Startling developments plunge the teenage-detectives into a dangerous chase by sea, air, and land in pursuit of a gang of hardened criminals who operate by the signs of the Zodiac. The Hardy's close pal Chet Morton tries to help them by using his newly acquired knowledge of astrology.

Tension mounts when the Flying Express vanishes - and so does Sam Radley, Mr. Hardy's skilled operative. Has Radley been kidnapped? Is he a prisoner aboard the stolen hydrofoil? Peril stalks Frank and Joe's every move as they hunt down the terrifying gangleader Zodiac Zig and his vicious henchmen.