On Amity Island, Chrissie Watkins (Susan Backlinie) leaves a late-night beach party to go skinny dipping in the ocean. Something violently grabs and pulls her underwater. The next day, Police Chief Martin Brody (Roy Scheider) and Deputy Hendricks (Jeffrey Kramer) find her remains washed up on shore. The medical examiner initially determines the cause of death was a shark attack. Brody orders the beaches closed, but Mayor Larry Vaughn (Murray Hamilton) overrules him, fearing news of a shark attack will ruin the summer tourist season, the town's primary revenue source. He rationalizes that there have been no prior shark problems in the area, and theorizes the girl was killed in a boating accident. The coroner concurs and Brody reluctantly accepts their conclusion.
A young boy is killed in another attack shortly after, and his mother offers a $3,000 bounty on the shark. Amid an amateur shark-hunting frenzy, local professional shark hunter Quint (Robert Shaw) offers his services for $10,000. Meanwhile, Matt Hooper (Richard Dreyfuss), a consulting oceanographer, arrives and examines the first victim's remains and declares it was a shark attack.
When a large tiger shark is caught, Hooper disputes it being the same predator, confirming this after finding no human remains inside its stomach. While Hooper and Brody search the local waters at night, they discover local fisherman Ben Gardner's half-sunken boat. Hooper examines the vessel underwater and retrieves a sizable great white shark's tooth embedded in the hull. Startled by finding Gardner's partial remains, Hooper drops the tooth. Brody and Hooper inform Vaughn that a great white shark is responsible, but he discounts their claims and refuses to close the beaches, allowing only added safety precautions. On the Fourth of July weekend, the beaches are packed with tourists. Following a false alarm, the shark enters a nearby estuary and kills a boater. Brody coerces a devastated Vaughn to hire Quint.
Quint, Brody, and Hooper set out on Quint's boat, the Orca, to hunt the shark. While Brody lays down a chum line, Quint waits for an opportunity to hook the shark. Without warning, it appears behind the boat. Quint, estimating its length at 25', harpoons it with a line attached to a flotation barrel, but the shark pulls the barrel underwater and disappears.
By nightfall, the men retire to the boat's cabin where Quint and Hooper compare scars and share how they received them. Quint recounts how he survived the shark attacks that followed the sinking of the warship USS Indianapolis during the War in the Pacific in 1945. The great white returns, ramming the boat's hull and killing the power. The men work through the night repairing the engine. In the morning, Brody attempts to call the Coast Guard, but Quint smashes the radio, enraging Brody. After a long chase, Quint harpoons another barrel into the shark. The men tie the line to the stern, but the shark drags the boat backwards, swamping the deck and flooding the engine compartment with seawater. Fearing the boat's transom will be pulled out, Quint cuts the line. He then heads toward shore to draw his quarry into shallow waters and suffocate it, but overtaxes and stalls the damaged engine.
With the boat immobilized, the trio attempt a riskier approach: Hooper dons scuba gear and enters the water in a shark-proof cage, intending to lethally inject the shark with strychnine using a hypodermic spear. The shark demolishes the cage, causing Hooper to drop the spear before escaping to the seabed. The shark attacks the boat directly, killing Quint. Brody, trapped on the sinking vessel, shoves a pressurized scuba tank into the shark's mouth, and, climbing the mast, shoots the tank with a rifle. The resulting explosion obliterates the shark. Hooper surfaces, and, clinging to boat wreckage, the two men paddle to Amity Island.