Two divers photograph the shipwrecked Orca underwater. A shark attacks, and the camera takes a photo as it drops to the sea floor. Police Chief Martin Brody rushes across town to attend the Amity Scholarship Fund benefit. Amity’s Mayor, Larry Vaughan, speaks at the ceremony. He introduces local developer, Len Peterson, who has donated $1500 to the scholarship fund. Chief Brody’s wife, Ellen, works for Len Peterson. Later in the evening, the shark shows up off of the coast of Amity. The next day, a local boater tells Chief Brody about an abandoned cruiser nearby, and Brody sends his deputy, Jeff Hendricks, to check it out. Local teenagers, including Brody’s son, Mike, go sailing. Back at the police station, Hendricks shows Brody the camera he found near the abandoned cruiser. They both think it strange that such an expensive boat was abandoned. As the teens have fun on the water, the shark swims below the surface. Further away, a teenage girl, Terry, jet skis as her mother drives their speedboat. When Terry gets pulled down by the shark, her mother turns the boat around to check on her. The shark jumps out of the water and takes a huge bite out of the boat. The mother shoots it with a flare gun and burns the shark’s face driving it away. Unfortunately, the boat also catches fire and explodes. Local teenagers Tina Wilcox and Eddie see the explosion from shore. Hendricks checks the wreckage but can’t find any bodies. When Brody questions the witnesses, they all swear one moment everything was fine then suddenly the skiers disappeared. Dragging the waters for the bodies, Hendricks accidently dredges up a power line. At home, Brody tells his son, Mike, to get a job and warns him to be careful while sailing with his friends. The next day, Tina and Eddie find a beached killer whale with a chunk bitten out of its body. Brody, Hendricks and marine expert, Dr. Elkins, examine the whole carcass. Brody suspects a great white shark attacked the whale, and tells Mayor Vaughan they may have another shark problem. While driving along the beach, Brody spots a piece of debris in the ocean. He wades out to retrieve it and discovers the burned remains of the speedboat driver. At the station, Brody puts cyanide in hollowed-out bullets and places them in his revolver. Back at home, he informs Mike he will be working the rest of the summer and is forbidden from sailing. The next day at the beach, Brody stands watch in the shark tower, while Mayor Vaughan, Peterson, and Ellen show potential real estate customers around the beach. Brody thinks he spots a shark and sounds the alarm, but after he rushes down to the water and shoots his gun at the predator, he realizes it is only a school of blue fish. The locals are traumatized, and Vaughan, Peterson, and Ellen are furious that Brody caused an unnecessary scene. That evening, Brody gets a call from Phil Fogarty, who has developed a roll of film from the camera Hendricks found, including a picture of the shark's eye. Believing he has the proof he needs to convince Vaughan, Brody rushes to town hall and shows Vaughan, Peterson and a few others the pictures. However, they don’t see what he sees, and Brody loses his temper trying to convince them. At a local restaurant, a pretty teenager named Jackie convinces Mike to go sailing to the lighthouse the next day, despite his father's orders. That night, Brody comes home drunk, and his wife realizes he’s just been fired, and Deputy Hendricks has been promoted to Chief. Early the next morning, Mike sneaks out to go sailing with his friends, but he is forced to bring his younger brother, Sean, so he won’t tell their parents. The teenagers sail off in several boats together. Mike, Andy and Jackie sail by a local diving instructor, Tom Andrews, and his class. The class does a lobster dive, but during the dive, Tom encounters the shark and returns to the surface too quickly. He suffers an embolism and is taken back to shore for medical attention. The shark makes its way toward the teenagers. On the water, Tina and Eddie are separated from the group. As they kiss on their boat, the shark swims into the boat, knocks Eddie into the water, and eats him. While driving into town, Brody and Ellen see an ambulance speeding toward the docks. Brody follows and talks to Hendricks and some of the diving students. A student claims Tom was scared by something in the water and panicked. Hendricks tells Brody and Ellen that he saw Mike sail out with the other teenagers. Alarmed, Brody and Ellen commandeer Hendricks's boat to find their son. Brody calls the Harbor Patrol and asks them to search for the teenagers. Brody, Ellen and Hendricks find Tina’s boat. When they ask her what happened, she sputters, “Shark!” Brody calls a passing boat and tells Ellen and Hendricks to take Tina back to shore. On his own, Brody takes the boat to find the teenagers. The shark attacks and Mike is knocked unconscious and thrown into the water. The teens panic and crash their boats into each other, but manage to pull Mike out of the water before the shark eats him. Two of the teenagers take the injured Mike back to shore. The ones left behind tie their boats together and attempt to drift to a body of land called Cable Junction. The Harbor Patrol reaches the teenagers, but as the helicopter pilot attempts to tow the teenagers, the shark jumps out of the water and pulls the helicopter underwater, killing the pilot. As Sean and Marge fall into the water, Marge saves Sean but gets swallowed by the shark. Jackie and Sean go into shock. The other teens have to coax Sean back to their boat. Hours go by and they see Cable Junction in the distance. Still searching for the teenagers, Brody spots Mike and his friends on their sail boat. Mike tells Brody where they were headed and that Sean is with them. Brody rushes to find them, but just as Brody reaches the teenagers the shark attacks Brody’s boat. He crashes the boat into the rocks while trying to escape, but manages to throw his hook line to the teenagers’ boat in order to pull them to shore. However, the hook gets caught on a power line and pulls it to the surface. The shark attacks the teenagers’ boat again, and they all fall into the water and scramble to swim to shore. Too scared to move, Jackie and Sean watch as the shark circles their boat. Brody gets the shark’s attention by beating the power line with an oar. The shark makes its way toward Brody, bites the power line, and gets electrocuted to death, sinking to the bottom of the ocean. Brody paddles out to Jack and Sean and takes them to shore, as the survivors reunite.