Nighttime on Amity Island. A bonfire rages as a group of teens gather around for an evening of fun, drinking, and the pursuit of each other. One of those pursuits involves a blond girl named Chrissie being chased by a guy who has caught her eye across the rising flames from the fire. His chase becomes even more intent when Chrissie starts disrobing while still running toward the shore. As she splashes into the water for a nude swim under the moonlight, the consumption of booze proves too much for her admirer who trips, falls and ultimately passes out before he can enjoy the fruits of temptation offered by a young naked woman in the ocean.
Oblivious to the fact that she will never be joined in her swim, Chrissie luxuriates in the surprising deep waters just off the shoreline. She is also oblivious to the fact that she has drawn the attention of another admirer who swims deep below, training his cold, hungry eyes upon the outline of her nude form. Suddenly Chrissie becomes aware of this new admirer as she is tugged viciously beneath the surface. She pops up again, aware now that she is fighting for her life against the tenacity of a great white shark. The shark takes stronger hold of Chrissie, easily dragging her this way and that across the surface of the ocean. And then, in an instant, the shark drags her down below the waves one last time.
Her remains are discovered by a deputy investigating a report of Chrissie’s disappearance, made by the young man too drunk to hear her screams of terror. The medical examiner suggests the cause of death might have been a shark attack and immediately Amity Island’s Chief of Police, Martin Brody, makes plans to create signs announcing the beach is closed. Mayor Larry Vaughn rushes to put the kibosh on this plan, since the ever-lucrative 4th of July influx of tourists from the mainland is imminent and even the merest whisper of a shark in the waters could kill the economy for the entire summer and, by extension, the entire year. When the coroner hesitantly admits that the girl might have been killed in a boating accident like the Mayor suggests, Brody knows he’s beaten.
He was right, however, as the examination of Chrissie’s remains by an oceanographic expert named Matt Hooper scientifically proves, and another shark attack that takes the life of a young boy horrifically confirms. Local shark fisherman Capt. Quint’s offer to catch and kill the shark for what seems an exorbitant sum is summarily declined, and a shark caught by one of the influx of amateurs drawn to Amity Island by much lower bounty is determined to be a tiger shark, far too small to be the beast causing all the ruckus.
Brody and Hooper set out on a night hunt which ends in the discovery of a tell-tale piece of evidence: the massive tooth of a great white shark. Unfortunately, Hooper drops this evidence upon being spooked by the unexpected appearance of the corpse of another victim. Nevertheless, they try to use this evidence, the dead fisherman and the lost tooth, to convince Mayor Vaughn to finally agree to close the beaches. He expressed how convenient it is that Hooper lost the only real evidence he has that a great white shark is responsible and stubbornly keeps the beaches open for the arrival of tourist money celebrating Independence Day.
Following a practical joke in which the dorsal fin of a shark proves to be a prank, the shark actually does show up and wreaks enough damage on enough people to finally spook the Mayor even more than Hooper was spooked by the underwater corpse. Brody uses this opportunity to push the Mayor to hire Quint now that his sum seems far less onerous. Quint is hired, but must submit to undesired terms: Brody and Hooper will be his only crew.
The trio set out on Quint’s fishing boat where time passes with no sign of the shark. Then, while Brody is complaining how he always gets the thankless job of tossing out the chum bait, the shark suddenly makes a dramatic announcement of its presence. This time it is Brody who is spooked; as he backs away from his very close encounter with the predator that is their prey, he lets Quint know exactly what they are in for by advising him that a bigger boat is going to be needed to catch this monster.
Having spotted the shark which is estimated to be somewhere between 20 and 25 feet long, Quint uses a harpoon gun to shoot lines into the thick hide of the shark that are attached to hollow barrels designed to float the shark up to the surface. One barrel fails to do the trick as the shark effortlessly pulls it down beneath the waves.
During a brief respite from the chase, the men drink and share stories about their scars. The lighthearted mood of good fellowship suddenly turns somber and dark as Quint relates the true story of the USS Indianapolis which sunk during the waning days of World War II without anyone having an idea of their location due to the highly secretive nature of their mission. Over the course of the next few days, most of those who survived the sinking either drowned or were eaten by sharks.
Morning comes, the shark reappears and the engines dies during the chase. As they try to repair the engine, Brody tries to call in for help from the Coast Guard but Quint maniacally destroys the ship’s radio with a baseball bat. The Great Shark Hunt continues with more barrels being shot into the fish and the agreement that Hooper should go down into the water in his shark cage and try to kill it through modern means.
Hooper goes into the cage. The cage goes into the water. The shark’s in the water and almost immediately attacks the cage, almost completely destroying it. Hooper manages to escape and swim away, but when the Brody and Quint bring the cage back up all they know is that it is mangled and there’s no sign of Hooper.
The two men don’t have time to ponder whether Hooper got away or not, much less conduct a search. The boat is rapidly gaining water and on the verge of sinking when their prey once again establishes his primacy as the sole predator. Literally coming out of the water and halfway onto what is left of the deck of Quint’s boat, the shark enjoys a last meal of the final victim of its assault on Amity Island.
With Hooper nowhere to be seen and presumed dead and Quint most assuredly dead inside the shark’s stomach, the task of turning the predator back into mere prey is left to the one member of the trio least likely to get the job done: the water-hating, land-lubbing, law enforcement officer whose entire body of knowledge about sharks has been accumulated in the short period of time that has passed since Chrissie started running toward the ocean with her clothes flying behind her.
On the verge of being eaten himself, Brody grabs one of the pressurized scuba tanks that Hooper brought aboard the boat and stuffs it through the shark’s wide open mouth. Then, climbing up the rapidly tilting mast, he takes aim with Quint’s rifle and shoots, managing to hit the tank dead center. Milliseconds later, the tank explodes inside the shark’s mouth, pulverizing the fish into a million pieces of sushi filet.
Seconds later, Hooper puts a small fright into Brody by rising to the surface behind him. Together the two use part of what remains of Quint’s fishing boat as ballast as they paddle back toward Amity Island.