In the aftermath of a hurricane, a series of strange events plague the coastal waters off Florida. People go into the water and never return. Corpses of killer whales drift ashore, ravaged from enormous bite marks. A fishing trawler is found adrift, with a mysterious gash in its hull.

Transferred to the coastal town of Merit, police officer Leonard Riker uncovers the horrible reality of an enormous Thresher shark lurking off the coast. Forty feet in length, it has taken a territorial claim to the waters near the town harbor. Armed with three-inch teeth, a scythe-like caudal fin, and unmatched aggression, the beast seeks to kill anything sharing the waters.