A Vietnam veteran with a psychic connection to sharks discovers their exploitation by the local aquarium and begins an underwater reign of terror to avenge them. The movie action scenes were shot using real sharks.
During the Vietnam War, US soldier Sonny Stein is saved from a pursuing enemy by a Mako shark. He begins to appreciate Makos after that. After the war, Stein finds work in the Philippine Islands as a marine salvager. A Filipino shaman gives him a medallion that helps him develop a telepathic rapport with Makos. Once back home in Florida, Stein decides to become Makos defender. A shady scientist who wants to research the sharks and a strip club owner who wants to use the sharks in a stripping act try to get Stein to use his powers to help them with their plans. Furious, Stein turns on the two men and things get ugly quickly for all of them.
Gifted with a mysterious talisman by a sage Filipino shaman, the shark-loving recluse, Sonny Stein, now possesses the unfathomable power to telepathically communicate with sharks. Having developed a mystical bond and deep appreciation for the agile oceanic predators, Sonny seems to prefer the company of sharks, and, especially, the short-fin Mako sharks; a unique ability that soon catches the attention of an unscrupulous scientist, and a cruel strip-club owner. And, before long, the fishermen of the community declare war on sharks, killing them by the dozens. Now, who can stop Sonny from exacting revenge on those who pose a threat to his sharks?—Nick Riganas
When scuba diver Sonny Stein (Richard Jaeckel) sees that one of his pet sharks has been hooked from a fishing boat above, he surfaces, climbs aboard, and kills the three men on board by throwing them overboard and allowing the sharks to eat them. Sonny then swims underwater to his own boat and speeds to a nearby marina to have a beer at a local eatery where he chats up with the friendly owner Butter (Milton 'Butterball' Smith).
Afterward, Sonny returns to his houseboat, moored on a nearby island, where Dr. Whitney (Ben Kronen), an ichthyologist at a local aquarium, waits with two local fishermen/henchmen, Pete (Harold 'Odd Job' Sakata) and Charlie (John Chandle). Dr. Whitney wants Sonny, who is known as the crazy "shark guy," to bring a female shark to Whitney's aquarium because he wants to film it giving birth. When Sonny balks at helping the aquarium, Dr. Whitney threatens to use his political influence to make sharks a "bounty fish," with an open hunting season. Sonny relents, saying he will deliver a pregnant shark, Matilda, but he wants her back at the end of the week. As the scientist leaves with Pete and Charlie, he tells them to keep watch on Sonny. After Dr. Whitney and the two fisherman leave, Sonny opens a trap door in his houseboat and talks to Sammy, one of the sharks swimming in the water beneath.
That evening, Sonny takes his boat back to the marina, and drives his van to the Rustic Inn, a lounge to drink a few beers. There, Karen (Jenifer Bishop), the owner's young wife, swims in a water tank behind two large windows over the bar. Barney (Buffy Dee), the owner, tells his bartender, Bob (Bob Gordon), that the customers are not spending enough time watching Karen, suggesting a more revealing swimming suit or nudity. A police deputy arrives talks about finding another empty, derelict fishing boat.
Later, outside the bar, Sonny sees Pete and Charlie harassing Karen as she drives away. The two men get into their van and follow her. A few minutes later, as Sonny drives toward the marina, he sees Karen's car alongside the road with a flat tire and Pete and Charlie pawing her on the ground, clearly attempting to rape her. Sonny stops, beats up the men, and gives Karen a ride. On their way, Sonny returns to the island to feed his sharks because they are on a strict schedule, and Karen joins him. There, Karen treats Sonny's fight injuries and admires a medallion with a shark's tooth that Sonny wears around his neck, and he explains why he never takes it off.
Years ago, in the Philippines, Sonny escaped from a couple of killers by diving into a shark-infested river and swimming to the other side, while the sharks attacked his pursuers. Afterward, a local shaman gave Sonny the medallion and said it would protect him from sharks as long as he wore it and took care of them.
Sonny opens the trap door and shows Karen his favorite shark, Sammy, swimming below, then briefly joins Sammy in the water to show her that it will not harm him. When Sonny finally takes Karen home, she is flirtatious, but he is too shy to respond. Inside, she tells Barney that Pete and Charlie drunkenly tried to rape her, but Barney brushes off the incident because the two men are among his best customers. She also tells her husband about Sonny and his shark.
After Barney swims in the lagoon behind their house, where he has installed a protective shark net, he returns to the house with the idea to buy Sonny's shark and make it a part of Karen's show at the lounge.
The next day, Barney arranges for Sonny to bring Sammy to the Rustic Inn and put it into the water tank. He assures Sonny that he has installed a clear, plastic barrier in the tank that separates Karen from Sammy. Sonny turns down a fee, saying that humans cannot own sharks, but Barney convinces Sonny to sign a paper for the sake of his insurance company. After Sonny leaves, Barney blasts high-frequency signals into the tank to make Sammy more animated. Meanwhile, Sonny takes Matilda, his female shark, to Dr. Whitney's aquarium.
Back at the marina, Sonny sees a fisherman displaying a large, dead shark in front of tourists, and notices that the fish was harpooned. He forces the fisherman to confess that he bought the shark for $20 from Pete and Charlie, who had several other dead sharks on their boat. Sonny gets into his scuba gear and sneaks up on Pete and Charlie's boat. He harpoons and kills both men, puts the engine in gear and jumps overboard, sending the boat crashing into a pier with Charlie hanging on a hook.
Later, at the Rustic Inn, Sonny notices that Sammy, behind the glass, is distressed and moving unnaturally. He smashes the high-frequency generator and attacks Barney, who claims he can do what he wants with Sammy because Sonny signed a bill of sale. Karen calls Sonny a freak and says she never wants to see him again.
Determined that he cannot trust anyone anymore, Sonny drives to Dr. Whitney's aquarium to rescue Matilda, but he finds her dead in Whitney's laboratory, along with several baby sharks. He throws Dr. Whitney into the aquarium's shark tank and watches the fish tear him to pieces.
Sonny returns to the Rustic Inn, sneaks into the tank area, and cuts a hole in the plastic that separates Sammy from Karen. That evening, he shows up at the crowded club for the opening of Karen's new act. As the window curtain opens above the bar, Sammy slips through the hole in the plastic barrier and attacks Karen. Moments later, police deputies arrive looking for Sonny, whom they allege is Dr. Whitney's killer. Sonny escapes into the water and rides with a shark, holding on to its fin. He arrives at his houseboat before the deputies and knocks them unconscious, but several vigilantes overwhelm him and pull off his medallion in the scuffle. Sonny drops through the trap door to escape, but without his protective medallion, the sharks turn on him and eat him.
