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A team of water skiers perform at Sea World theme park, narrowly avoiding the great white shark swimming just below the surface. On shore, spokesman Leonard Glass briefs reporters on the park’s latest attraction, the Undersea Kingdom, a structure comprised of four viewing tunnels, a control center, and the Neptune Room restaurant and lounge, all conceived by park owner Calvin Bouchard. The attraction lies forty feet below the surface of a man-made lagoon, connected to the ocean by deep-water channels. Meanwhile, engineer Mike Brody instructs his crew to secure a severely damaged gate that connects the lagoon to the ocean, before meeting his girl friend, marine biologist Kathryn Morgan, for a dinner date. At the park exit, they encounter Mike’s brother, Sean, who has just arrived from Colorado, and he joins them at a local tavern. That evening, technician Shelby Overman dives into the lagoon to secure the gate, but is killed by the great white shark before his work is finished. At the tavern, his girl friend, waitress Charlene Tutt, asks Mike about Shelby’s whereabouts, suspecting her boyfriend of having an affair. Mike, however, is more concerned that Shelby may have defied orders barring overtime work. Sean joins a game of Standoff with Kelly Ann Bukowski, a member of the ski team, and a romance develops between them. Kelly offers Sean a ride home, then tries to coax him into the surf for a late night swim, but Sean refuses, citing his fear of the ocean. While driving Kathryn home, Mike tells her of the shark attacks he and his brother witnessed as children on Amity Island, where their father was chief of police. Sean explains that he moved to Colorado to get as far away from the ocean as possible. They come upon Kelly’s car and Mike is amazed to find his brother playing with Kelly in the surf. The next morning, Kathryn arrives at the park and learns that the dolphins, Cindy and Sandy, are disobeying their trainers and displaying nervous behavior. Kathryn later discourages the advances of Philip FitzRoyce, a famous English documentarian hired by Calvin to publicize the park’s new attraction. Elsewhere in the park, Calvin orders Mike to fire Shelby for leaving the gate open, and Charlene is angry over her boyfriend’s prolonged absence. When Mike discovers that Shelby has left behind his wallet and identification, he suspects the worst and asks Kathryn to join him in a search of the lagoon. During their exploration, they are threatened by a small great white shark, but are pulled to safety by Cindy and Sandy. Calvin, Philip, and his assistant, Jack Tate, are told of the shark’s presence, and Philip suggests killing the beast. However, Kathryn believes it would be to Calvin’s advantage to have the only great white shark in captivity, and convinces him to take it alive. As they prepare for their mission, Philip suggests bringing a pair of hand grenades along as a last resort. Both Mike and Calvin disapprove, saying that an explosion could easily damage the acrylic tunnels. Kathryn, Philip, and Jack enter the lagoon armed only with tranquilizer darts, while Mike observes from the surface. Within moments, the shark attacks Kathryn from behind, gripping her oxygen tank in its teeth. After Philip frees her, Mike fires a dart into the shark’s back and renders the creature helpless. Days later, visitors crowd the park for the opening of the Undersea Kingdom, and Calvin orders the shark to be transferred to a pool for display, dismissing any suggestion that he consult Kathryn beforehand. When Karthryn learns of the transfer, she and her assistant, Dan, hurry to the pool and find the shark dead. Meanwhile, tourists visiting the undersea kingdom are horrified as Shelby’s mutilated body floats past. Kathryn examines the body and discovers a bite three feet across, indicating the presence of a thirty-five foot shark in the lagoon, which she believes is the mother of the captured shark. As she and Mike deliver the news to Calvin, Philip, and Jack in the lounge, the shark appears on the other side of the glass partition. Calvin orders the park evacuated and closed, while Mike warns the ski team and tourists of the impending danger. The shark damages one the underwater tunnels, trapping a large group of visitors inside as automatic doors contain the incoming water. Mike and his crew work feverishly in the machine shop, fabricating parts to repair the tunnel. Philip and Jack approach Calvin with a plan to eliminate the shark by trapping it inside a filtration pipe, using the powerful suction of the water pumps, while capturing the event on film. Calvin observes from the control center as Jack secures Philip’s lifeline, and the shark is successfully trapped inside the pipe. However, when the lifeline fails to hold, the suction draws Philip into the shark’s gaping mouth and he is swallowed whole. Calvin assumes the danger has passed and shuts off the pump, allowing the shark to escape. Kathryn joins Mike in the lagoon as he patches the tunnel, as the shark looms nearby. However, the dolphins intercede and create a distraction, allowing the couple to return safely to the control center. Inside the tunnel, the water recedes, the doors open, and the tourists are able to leave. After Kathryn and Mike return to the control center, the shark crashes through the observation window and kills Calvin's technician nephew, Fred, while water pours into the room. With its head caught in the jagged glass, the shark thrashes helplessly as Calvin and another technician escape, and Mike glimpses Philip’s lifeless hand holding a grenade inside the creature’s mouth. Using a metal rod, Mike removes the pin, and he and Kathryn take cover before the shark’s head explodes. The couple swims to the surface, flanked by Cindy and Sandy.