This trendsetting creature thriller begins when a girl named Chrissie Watkins leaves a beach party at dusk on Amity Island and goes skinny dipping. While swimming out near a buoy, she is seized by something from below; it thrashes her around and drags her underwater.
Chrissie is reported missing and her remains are later found on the beach by the Deputy of police chief Martin Brody. The medical examiner informs Brody that she was killed by a shark. Brody plans to close the beaches but is overruled by mayor Larry Vaughn, who fears that reports of a shark attack will ruin the summer tourist season, the town's primary source of income.
The medical examiner consequently attributes the death to a boating accident. Brody reluctantly goes along with the explanation. The shark then kills a young boy swimming at the beach. His mother places a bounty on the shark, sparking an amateur shark-hunting frenzy and attracting the attention of local professional shark hunter Quint, who offers to kill the shark for $10,000.
Marine biologist Matt Hooper examines Chrissie's remains and determines that she was killed by a shark, not a boat. A large tiger shark is caught by fishermen, leading the townspeople to believe the problem is solved. Hooper asks to examine its stomach contents, but Vaughn refuses.
That evening, Brody and Hooper secretly open the shark's stomach and discover that it does not contain human remains. They head out to sea to find the shark, but instead find the wreckage of a boat belonging to local fisherman Ben Gardner. Hooper explores the vessel underwater and discovers a sizable shark's tooth protruding from the damaged hull before he is startled by Gardner's corpse, causing him to drop the tooth.
Despite Brody and Hooper's protests, without evidence, Vaughn refuses to close the beaches. Many tourists arrive on the Fourth of July. A children's prank causes panic at the main beach while the shark enters a nearby estuary and kills a man. Brody's son Michael, who narrowly escapes the attack, goes into shock. Brody persuades Vaughn to hire Quint, and Quint reluctantly allows Hooper and Brody to join the hunt. The three set out to kill the shark aboard Quint's vessel, the Orca.
Brody is given the task of laying a chum line but an enormous great white looms up behind the boat, and the trio watch it circle the Orca. Quint estimates its size at twenty-five feet in length, with a weight of three tons. He harpoons it with a line attached to a flotation barrel, but the shark pulls the barrel underwater and disappears.
The men retire to the cabin, where Quint relates his experience with sharks as a survivor of the sinking of the USS Indianapolis. The shark returns, damages the hull and slips away. It reappears in the morning. Brody attempts to call the U.S. Coast Guard, but Quint destroys the radio.
Quint harpoons two more barrels to the shark, and the men tie them both to the stern, but the shark drags the boat backwards, forcing water onto the deck and flooding the engine. Quint heads toward shore, hoping to draw it into shallow waters and suffocate it. In his obsession to kill the shark, Quint burns out the Orca's engine.
With the boat immobilized, the trio attempt a desperate approach: Hooper dons scuba gear and enters the ocean inside a shark proof cage, intending to stab the shark with a hypodermic spear filled with strychnine. The shark attacks the cage from behind, causing Hooper to drop the spear.
When the shark becomes entangled in the wrecked cage, Hooper escapes and hides in the seabed. The shark then leaps onto the boat, crushing the transom. Quint slides down the deck and is eaten by the shark. When the shark attacks again, Brody shoves a pressurized scuba tank into its mouth, then takes Quint's rifle and climbs the sinking Orca's mast.
The shark, with the tank still in its mouth, begins swimming toward Brody. Brody shoots the tank, causing it to explode, blowing the shark to pieces. Hooper resurfaces and he and Brody use the barrels as flotation aids as they swim toward the shore.mouse