One summer evening in late June on the New England island of Amity, teenager Chrissie Watkins invites a drunken fellow student, Cassidy, to skinny dip in the ocean. Although enthused, Cassidy passes out a few feet from the shore, while Chrissie strips and dives into the sea only to be brutally attacked from underwater. The next morning, police chief Martin Brody meets Cassidy, who has reported Chrissie missing, on the beach just as Deputy Hendricks discovers the mutilated remains of a female body. Suspecting that Chrissie was a victim of a shark attack, Brody hurries to his office to make out a report and consult with the town physician. Determined to close the beaches when the doctor confirms his fears, Brody sets off to Amity Bay, but is intercepted by Mayor Larry Vaughn, two city council members and the doctor. Vaughn reminds Brody that closing the beaches requires a signed city ordinance and that the Fourth of July weekend is about to begin. When the doctor reluctantly admits that the body may have been mutilated by a motorboat blade and Vaughn insists they do not want to start a pointless panic, Brody grudgingly agrees to keep the beaches open. The next day, an uneasy Brody oversees the crowded beach, accompanied by his wife Ellen and their two young sons, Michael and Sean. Dozens of children and young people thrash about in the surf and a dog repeatedly fetches a stick thrown in the water by his owner. Moments later, however, the dog disappears and a group of people suddenly notice a pool of bloody red foam in the sea. As the swimmers and waders run to the beach in a panic, a mangled raft washes to shore while vacationer Mrs. Kintner searches in vain for her young son, Alex. After Mrs. Kintner posts a three-thousand-dollar reward to kill the shark that killed Alex, Brody and the Amity city board meets with local businesses, fisherman and townspeople to quell their mounting alarm. When Brody acknowledges that he must close the beaches, Vaughn reassures the dismayed business owners that the closure will last only twenty-four hours. The meeting is interrupted by local professional fisherman and shark hunter, Quint, who vows to capture the shark single-handedly for $10,000, which Vaughn agrees to consider. The following morning, Brody is horrified to find Amity harbor teaming with boats and people from Connecticut, Rhode Island and New Jersey who have responded to Mrs. Kintner’s reward offer. Struggling to control the crowds who bear everything from dynamite to guns to small fishing reels, Brody is relieved when Matt Hooper from the Oceanographic Institute arrives. At police headquarters, Hooper examines Chrissie’s remains and declares that the wounds are from a sizable shark. That afternoon a group of fishermen triumphantly return to Amity harbor with the carcass of a ten-foot shark which they proudly display for reporters and locals. Although Vaughn is delighted by the exhibition, Hooper insists the bite radius of a Tiger shark is too small to be the same shark that killed Chrissie. As Brody remains doubtful, Mrs. Kintner arrives and demands to know why he allowed the beaches to remain open after Chrissie’s death. That evening, Hooper visits the brooding Brody at home and reaffirms that the captured shark is not the one that killed Chrissie, and presses the chief to allow him to cut open the captured dead shark to explore its digestive remains. After Hooper determines that the shark caught by the fishermen has no human remains inside it, Brody realizes that he must close the beaches, but Hooper insists they immediately go in search of the killer shark in his high-tech exploration boat, the Aurora. Despite Brody’s frank admission that he fears the water, Hooper forces the chief to accompany him. With the aid of the Aurora's powerful spotlights, Brody and Hooper soon come upon a half-sunken dinghy showing unusual signs of damage and Brody recognizes the boat as belonging to an islander. Donning scuba gear, Hooper goes underwater to inspect the little boat’s hull and pulls an enormous shark tooth embedded in the planking. When the mangled remains of a torso abruptly float by a gaping hole in the boat, the startled Hooper drops the tooth. The next morning, Brody and Hooper met Vaughn on the beach to excitedly report that the shark attacks were made by a Great White. Without the tooth as evidence, however, the mayor remains skeptical and insists the beaches remain open the next day, which is the Fourth of July. The holiday dawns to hordes of vacationers packing the beaches. Hooper abandons a commitment to an eighteen-month research project in order to search for Amity’s Great White shark, while Brody, Hendricks and backup deputies with helicopter support observe the waters. Distressed that no one has actually gotten into the water, Vaughn appeals with a family to do so and soon the surf is teaming with people. When Brody’s son Michael asks permission to take his new sailboat out to sea, Brody pleads with him to go into the nearby estuary. While Vaughn cheerfully gives an interview to a television reporter, swimmers are suddenly terrified to see a large fin cutting across the water. As the panicked crowd returns en masse to the beach, Brody’s assistants reveal the fin to be a hoax perpetrated by two local teenage boys. Meanwhile, a young woman standing between the sea and the pond sees a massive underwater form head into the relatively shallow estuary where Michael and his friends are struggling to raise their sail. Nearby, a man in a dinghy calls advice to the boys just as the underwater creature smashes into his boat. The subsequent swell overturns Michael’s small sailboat and, as the boys thrash about, Michael witnesses the man being bitten in half by the enormous shark and faints. Meanwhile, the young woman’s continued cries alert Brody who races toward the pond as Michael’s friends pull him safely to shore. Later, at the hospital, where Michael is declared fine, a stunned Vaughn wonders if he can be held accountable for keeping the beaches open, but an angry Brody forces him to sign a contract hiring Quint. The next day, Brody and Hooper meet Quint at his pier-side office where Brody officially charters the fisherman’s boat, the Orca. Although Quint chafes about the college educated Hooper joining them, Brody insists that the oceanographer and much of his technical equipment be taken on board. Over the next couple of days, the Orca roams far out to sea in search of the shark. One afternoon Quint’s thick cable fishing line is bitten in two, but otherwise their quarry remains elusive. Soon after, as a grumpy Brody resumes shoveling bloody chum out to sea to lure the shark, the creature breaks the surface of the water, its massive mouth gaping. Stunned by the enormity of the shark, Brody staggers into the cabin and tells Quint that he will need a bigger boat. As Quint and Hooper excitedly watch the shark circle the Orca, the older man declares the creature is at least twenty-five feet long and three tons. While Quint prepares to shoot a cable line attached to a flotation barrel into the shark, Hooper attaches a radio tracking device to the barrel. After striking the shark with the harpoon and cable, the Orca follows the racing barrel, but Quint is taken aback when the shark easily pulls the air-filled keg underwater and disappears. Night falls with no further sign of the shark and the men sit in the tiny cabin drinking and talking. Quint reveals that in World War II, he served on board the U.S.S. Indianapolis which was sunk by a Japanese submarine and nearly eight hundred of its surviving crew was lost to shark attacks while waiting for rescue in the open sea. The men fill the subsequent tense silence with songs, when the shark surfaces in the dark and rams into the hull, damaging the boat’s shaft. Despite Quint firing several rounds at the shark, it remains unaffected, but disappears for the remainder of the night. The next morning, Quint and Hooper struggle to repair the battered rudder and engine housing, when the shark surfaces and Quint shoots another cable and barrel into it, then ties the cables lines to the transom cleats. As the shark, now hooked to two floatation barrels, races further out to sea, Quint pushes the rough running engine of the Orca in pursuit, ignoring Brody’s argument to turn back toward land. Later the shark appears to have vanished, only to surface suddenly and attack the cable lines. Panicked, Brody attempts to radio the Coast Guard, but Quint smashes the radio with a bat. Quint then calmly shoots another line and a third barrel into the shark, but when the shark heads to sea again towing the Orca, Quint is forced to cut the taunt cable lines, fearing that the transom will be pulled off. As the battered and listing Orca begins taking on water, the men watch incredulously as the barrels turn toward them, then submerge and go beneath the boat. Moments later, the shark rams the keel. The ship’s stressed engine bearings begin to smoke, and Quint, masking his concern, pushes the engine as the shark begins pursuing them. Upon reaching the boat, the great shark rises up, biting into the transom. The violence of the creature’s attack finishes the Orca's weakened engine. The shark disappears as Brody and Hooper realize that the Orca is sinking by the stern. Handing Brody a lifejacket, Quint asks Hooper about the shark cage and other equipment he has brought on board. When Hooper reveals that he has a large syringe full of strychnine nitrate, Quint declares the syringe will never penetrate the shark’s tough skin. Hooper nevertheless volunteers to go underwater in the cage and attempt to shoot the syringe into the shark’s mouth with the harpoon gun. Despite Brody’s protests, Hooper dons scuba gear and oxygen, and is lowered in the cage into the water. Within moments the shark appears and rams the cage from behind Hooper, then grabs the bars and shakes the cage, causing the terrified Hooper to drop the harpoon gun, unfired. Fleeing the shark’s crazed attack through the mangled cage bars, Hooper swims to the sea bottom. Meanwhile the shark, momentarily trapped between the cage and the side of the Orca, thrashes violently as Quint struggles to crank the winch. The bent ginpole gives way as the shark extricates itself and Quint and Brody are horrified when the battered, empty cage surfaces. The shark reappears at the stern and again lunges at the Orca's deck, tilting the boat sharply, causing Quint and Brody to tumble and slide toward the maddened creature. Brody hangs on to the cabin doorframe, but Quint, unable to maintain his grip on Brody’s legs, slides directly into the shark’s jaws. When the shark submerges with Quint’s bloodied corpse, Brody casts about for a weapon and spots Hooper’s remaining oxygen tank. When the shark attacks again, Brody manages to wedge the tank into its mouth. Taking Quint’s rifle, Brody climbs out onto the bridge mast, which is now almost parallel with the water, and as the shark comes at him, fires repeatedly until a bullet strikes the oxygen tank, causing it to explode and blow off the creature’s head. As blood and flesh rain down on Brody and the nearly submerged Orca, the shark’s other half falls slowly through the water. Moments later, Brody is amazed when Hooper surfaces. The men laugh weakly in relief and, after Hooper learns of Quint’s demise, the men use the remaining floatation barrels as support and paddle their way toward land.