One night, after a beach party, Chrissie Watkins (
Susan Backlinie) goes skinny-dipping in the ocean off of Amity Island, a peaceful beach community that relies on summer tourism and fishing for its economic survival. While another partygoer lies drunk on the beach, Chrissie is attacked by something unseen that comes at her from underneath the water. As she screams in terror and pain, it drags her back and forth in the water before pulling her underneath for good.
Martin Brody (
Roy Scheider), the new Chief of Police for Amity Island sits down at his typewriter, while the police secretary relays the crime reports of the day. While she rambles on about a complaint that the children of Amity have been "karate-chopping the picket fences," Chief Brody receives a phone call form the coroner. After hanging up, he finishes typing up the cause of death in Chrissie's case: "SHARK ATTACK."
The next day, Brody and his deputy begin painting signs and puts them up at the beach to close it off to the bathers. Brody's intentions are quickly noticed by the Mayor Vaughn (
Murray Hamilton) and the rest of the business owners of the island. Talking to Brody down by the dock, Mayor Vaughn gets the coroner to take back his original claim of "shark attack" as the cause of death, and strong-arm Brody into reopening the beaches in anticipation of the impending summer influx of "main landers." Brody reluctantly relents, after the mayor and the coroner persuade him that Chrissie could have been struck and killed by a boat propeller. Mayor Vaughn and other civic leaders want to avoid a panic that would keep tourists from flocking to Amity.
To the delight of tourists and the business owners that depend on them, the next day brings the islanders and main landers alike to the beach - enjoying the summer weather. Chief Brody sits watching the surf, when an older man in a swim cap comes up and reveals that Brody is afraid of the water. Annoyed, Brody replies, "That's some bad hat, Harry."
While the swimmers frolic in the water, a boy splashes along on a raft. Suddenly, he's dragged under, and red blossoms in the water. Brody sees this, and stands up and yells for everyone to get out of the water. A mass exodus from the water leaves one lone mother walking up and down the surf yelling for her son Alex. His raft drifts up on the beach - shredded and bloody.
Brody greets Matt Hooper (
Richard Dreyfuss), at the dock. After the recent attack, Brody had decided to call the mainland Oceanographic Institute for their expert advice - Hooper has arrived to investigate. Meanwhile, the death of Alex Kitner breathes life back into the shark panic; the boy's mother places a bounty on the shark, encouraging amateur shark hunters from all over New England to descend on Amity and crowd the harbor with boats and unsafe fishing methods to catch the shark that killed Alex.
Brody and the law enforcement have their hands full while the anyone with a boat is frantically trying all methods to catch the shark and collect the bounty. When some local fishermen catch a tiger shark, all of the townspeople are overjoyed at the thought that the shark terror is over. The mayor and all declare the crisis officially over and demand that the beaches be reopened. Hooper objects, saying that the bite marks do not match the wounds on Chrissie, and asks to open up the shark and see if the remains of Alex Kitner are among the contents of the shark's stomach. The mayor quickly squashes the idea of having a semi-digested boy "spilling out on the dock," and the Chief and Hooper have to retreat.
Unable to let it go, Hooper visits Brody for dinner that night. Brody has clearly decided to get drunk on wine, while Hooper convinces him to go down to the dock, break into the boat house and open up the shark once and for all. With the nagging doubt still in his brain, Brody agrees; they open the shark, and all they find is a license plate that the shark has eaten. This shark is clearly not their culprit. Hooper feels that the real shark is most likely a Great White.
Continuing their investigation, Hooper and Brody now decide to take a boat out into the harbor, where the police had recently found a derelict boat of a local fisherman. Hooper goes underwater to investigate further and discovers a great white tooth lodged in the boat's hull. Hooper loses the tooth when he is startled by the fisherman's severed head. Without the tooth, he and Brody are unable to persuade Mayor Vaughn that Amity still has a serious shark problem.
On July 4th, the shark attacks and kills a boater, an event that nearly claims the life of Brody's eldest son. Brody finally convinces Mayor Vaughn to hire an eccentric local fisherman, Quint (
Robert Shaw), to hunt and kill the shark. Quint, Hooper, and Brody set out on Quint's ship, the Orca. To attract the shark, they throw bloody fish remains into the water, and when it appears, it shocks all of the men with its sheer size; it is indeed a Great White, and they estimate that it is 25 feet long. Quint is at first disdainful of Hooper and the scientific equipment he brings on board but after the two men drunkenly compare fishing wounds, a bond develops between the men. The shark proves to be smarter than Quint and Hooper expected and soon has disabled the boat. Hooper goes down in a shark cage in an attempt to jab it with a poisoned explosive dart. However, the shark attacks Hooper's cage and he barely escapes with his life. The shark then leaps out of the water and onto the Orca's transom, driving it beneath the waves. Quint slides down the deck and into the waiting shark's mouth. Brody is able to lodge one of Hooper's airtanks into the shark's jaws and detonates it with a well-aimed rifle shot as the Orca sinks around him. As the shark's corpse sinks to the ocean floor, Brody and Hooper paddle back to shore.