The film begins at a late night beach party on Amity Island, from which a young woman named Chrissie Watkins (Susan Backlinie) leaves to go swimming. She strips naked on the beach and dives into the water. While in the water, she is suddenly jerked around and then pulled under by an unseen force. The next morning, Amity’s new police chief Martin Brody (Roy Scheider) is notified that Chrissie is missing. Brody and his deputy Len Hendricks (Jeffrey C. Kramer) find her mutilated remains washed up on the shore. The medical examiner informs Brody that the victim’s death was due to a shark attack. Brody, who fears the ocean, heads out to close the beaches, but is intercepted and overruled by the town mayor Larry Vaughn (Murray Hamilton), who fears that reports of a shark attack will ruin the summer tourist season which is the town’s major source of income. The medical examiner says he was wrong about a shark attack and tells Brody that it was a boating accident. Brody reluctantly goes along with this.
A week later, a young boy named Alex Kintner is attacked and eaten by a shark while swimming off a crowded beach. Mrs. Kintner places a $3,000 bounty on the animal, sparking an amateur shark hunting frenzy and attracting the attention of local professional shark hunter Quint (Robert Shaw). Quint interrupts a town meeting to offer his services; his demand for $10,000 is taken “under advisement”. Brought in by Brody, marine biologist Matt Hooper (Richard Dreyfuss) conducts an autopsy on Chrissie’s remains and concludes she was killed by a shark. A large tiger shark is caught by a group of novice fishermen, leading the town to believe the problem is solved, but an unconvinced Hooper asks to examine the contents of the shark’s stomach. Because Vaughn refuses to make the “operation” public, Brody and Hooper return after dark and learn that the captured shark does not contain human remains, just fish and garbage. Scouting aboard Hooper’s state-of-the-art boat, they come across the half-sunken wreckage of local fisherman Ben Gardener’s boat. Hooper dons a wetsuit and discovers a giant shark tooth and another victim, Gardener (this causes Hooper to drop the tooth). Vaughn still refuses to close the beach; on the Fourth of July the beaches are covered in tourists. While a prank triggers a false alarm and draws the authorities’ attention, the real shark enters an estuary, kills a life guard, and nearly kills Brody’s oldest son Michael. Brody forces a stunned Vaughn to hire Quint. Brody and Hooper join the hunter on his boat, the Orca, and the trio set out to kill the man-eater.
At sea, Brody is given the task of laying a chum line, while Quint uses a large fishing pole to try to snag the shark; the first results are inconclusive. Quint suspects shark, Hooper, a game fish. As Brody continues his task, the enormous shark suddenly looms up behind the boat. After a horrified Brody announces its presence (”You’re gonna need a bigger boat!”), Quint and Hooper watch the great white circle the Orca and estimate that the new arrival weighs 3 tons (2.7 metric tonnes) and is 25 feet (8m) long. Quint harpoons the shark with a line attached to a flotation barrel, designed to prevent the shark from being able to submerge as well as to track it on the surface; but the shark pulls the barrel under and disappears. Night falls without another sighting, so the men retire to the boat’s cabin where Quint and Hooper compare their various scars and Quint tells of his experience with sharks as a survivor of the World War II sinking of the USS Indianapolis. The shark reappears while the men sing, damages the boat’s hull, and slips away before the men can harm it. In the morning, while the men make repairs to the engine, the barrel suddenly reappears at the stern. Quint destroys the radio to prevent Brody from calling the Coast Guard for help. The shark attacks again, and after a long hard chase, Quint harpoons it to another barrel. The men tie the barrels to the stern; but the shark drags the boat backwards, forcing water onto the deck and into the engine, flooding it. Quint harpoons it again, attaching three barrels in all to the shark, while the animal continues to tow them. Quint is about to cut the ropes with his machete when the cleats are pulled off the stern. The shark continues to attack the boat and Quint powers towards shore with the shark in pursuit, hoping to draw the shark into shallow waters where it will be beached and drowned. In his obsession to kill the shark, Quint overloads his damaged engine, causing it to explode.
With the Orca immobilized, the trio try a desperate approach; Hooper dons his scuba gear and enters the ocean inside a shark proof cage, intending to stab the shark in the mouth with a hypodermic spear filled with strychnine nitrate. The shark instead destroys the cage, causing Hooper to lose the spear and flee to the seabed. As Quint and Brody raise the remnants of the cage, the shark throws itself onto the boat, crushing the transom and causing the boat to begin sinking. Quint slides into the shark’s mouth, slashing at it in vain with his machete, before being pulled under and devoured. Brody retreats to the boat’s cabin, which is now partly submerged, and throws a pressurized air tank into the shark’s mouth when it rams its way inside. Brody takes Quint’s M1 Garand rifle and climbs the rapidly-listing mast of the boat where he temporarily fends off the attacker with a harpoon. The shark circles around and charges one last time at Brody, who is now only a foot or so above the water. Brody starts firing at the air tank still wedged in the shark’s mouth. He says “Smile you son of a bitch!”, and scores a hit, and the highly pressurized tank blows the shark’s head to pieces and sends the rest of its body to the bottom of the ocean in a massive cloud of blood. Hooper surfaces and reunites with Brody, whereupon the two survivors use the leftover barrels to construct a makeshift raft and paddle back to Amity Island.
