Four years on from the horrible shark attacks which plagued Amity Island, Police Chief Brody (Roy Scheider) is still uneasy about the prospect of another shark visiting the bay. So when the film developed from the camera of a missing diver shows what looks like a shark fin, he begins to get very worried indeed. However, Mayor Larry Vaughn (Murray Hamilton) again plays down the incident, and when Brody raises a panicked false alarm from his observation tower on the beach, the mayor even goes as far as firing him. It is then that the bloodbath really begins.