In the New England beach town of Amity Island, a young woman goes for a late-night ocean swim during a beach party. An unseen force attacks and pulls her underwater. Her remains are found washed up on the beach the next morning. After the medical examiner concludes it was a shark attack, newly hired police chief Martin Brody closes the beaches; Mayor Larry Vaughn persuades him to reconsider, fearing the town's summer economy will suffer. The coroner, apparently under pressure, now concurs with the mayor's theory that it was a boating accident. Brody reluctantly accepts their conclusion until a young boy, Alex Kintner, is killed at a crowded beach. A bounty is placed on the shark, causing an amateur shark-hunting frenzy. Quint, an eccentric and roughened local shark hunter, offers his services for $10,000. Consulting oceanographer Matt Hooper examines the girl's remains, confirming that an abnormally large shark killed her.
When local fishermen catch a tiger shark, the mayor declares the beaches safe. Mrs. Kintner confronts Brody and blames him for her son's death. A skeptical Hooper dissects the tiger shark and, finding no human remains inside its stomach, determines a larger shark killed the victims. While searching the night waters in Hooper's boat, Hooper and Brody find the half-sunken vessel of Ben Gardner, a local fisherman. Underwater, Hooper removes a sizable shark tooth from the boat's hull, but accidentally drops it after discovering Gardner's severed head. Vaughn dismisses Brody and Hooper's assertions that a huge great white shark caused the deaths, and refuses to close the beaches, allowing only increased safety precautions. On the Fourth of July weekend, tourists pack the beaches. The shark enters a nearby lagoon, killing a boater and coming close to killing Brody's son. Brody then convinces a guilt-ridden Vaughn to hire Quint.
Despite tension between Quint and Hooper, they and Brody head to sea on Quint's boat to hunt the shark. As Brody lays down a chum line, the shark suddenly appears behind the boat. Quint, estimating it is 25 feet (7.6 m) long and weighs 3 tonnes (3.0 long tons; 3.3 short tons), harpoons it with a line attached to a flotation barrel, but the shark pulls the barrel underwater and disappears.
At nightfall, Quint and Hooper drunkenly exchange stories about their assorted body scars. One of Quint's is a removed tattoo, and he reveals that he survived the attack on the USS Indianapolis, during which many US sailors were killed by sharks. The shark returns, ramming the boat's hull and disabling the power. The men work through the night, repairing the engine. In the morning, Brody attempts to call the Coast Guard, but Quint, obsessed with killing the shark without outside assistance, smashes the radio. After a long chase, Quint harpoons the shark with another barrel. The line is tied to the stern cleats, but the shark drags the boat backward, swamping the deck and flooding the engine compartment. As Quint is about to sever the line to save the boat's transom, the cleats break off. The barrels stay attached to the shark. To Brody's relief, Quint heads toward shore to draw the shark into shallower waters, but the overtaxed engine fails.
As the boat takes on water, the trio attempts a riskier approach. Hooper suits up and enters a shark-proof cage, intending to lethally inject the shark with strychnine via a hypodermic spear. The shark viciously attacks the cage, causing Hooper to drop the spear. While the shark destroys the cage, Hooper escapes to the seabed. The shark leaps onto the boat's stern, subsequently devouring Quint. Trapped on the sinking vessel, Brody shoves a scuba tank into the shark's mouth and, climbing onto the crow's nest, shoots the tank with a rifle. The resulting explosion kills the shark. Hooper resurfaces and he and Brody paddle back to Amity Island, clinging to the remaining barrels.
