In the north woods of Wisconsin, college student Peter Van Clease comes to stay at his family’s fishing lodge. He is accompanied on this summer jaunt by four of his college friends: Ann Colbert, Kiersten, Rodney and Finner. They arrive just before the start of the “Muskie Madness” fishing contest that brings thousands of tourists to the area. Top prize for the catching the largest fish is $5,000. When Peter and his friends get to the family lodge, caretaker Wayne Duerst is openly hostile toward them while Wayne’s grandson, Evelyn Duerst, a Vietnam Vet, spouts a crazy rant. The five friends wonder about what they’ve gotten into. When Peter goes out to the lodge’s dock, the chirping of a cricket-like insect known as the cicada reminds him of his grandfather’s disappearance. Seventeen years ago, Peter’s grandfather was fishing on that dock when a fishing hook caught his face and pulled him into the lake, never to be seen again. When the group goes to dinner at a lakeside restaurant, a woman on the dock is caught by a fishing hook and pulled into the water. Meanwhile, in a fishing shack, local fisherman Denny Dobyns has caught an extra-large fish, one that can likely win the contest. Denny keeps the fish in a barrel and puts a tracking tag on it, so he can pretend to catch it during the contest. However, Wayne Duerst overhears Denny plotting and drops the fish back into the lake after Denny leaves. The next morning, Rodney and Finner go out fishing in a boat. Finner, who brought his own fishing rod, catches Denny’s extra-large fish. Inspired, Rodney wants to enter the contest and goes to Leroy Leudke’s bait shop to get fishing equipment. Leudke tells the story about how Peter’s grandfather and Wayne Duerst used to catch trophy fish by reeling them in, then shooting them in the head with a Cooper bullet. After the fish was dead, they would dig the bullet out of its head and reuse it to kill the next fish. They killed thirty to forty fish that way until one day Duerst accidently shot Peter’s grandfather in the back with that bullet. Rodney goes out in the speedboat he rented. However, as he enjoys the sun, he is hit by a fishing hook that drags him into the water. Later, Peter, Ann and Kiersten are drinking beer at a restaurant when the sheriff brings an empty boat to shore. They found blood and M-16 rifle bullets in the boat and worry about Rodney’s whereabouts. Peter suggests they talk to Wayne Duerst about the bullets. When they go to question Duerst, he gives them his rifle, but it is not an M-16. Peter demands the sheriff investigate Rodney’s disappearance, believing he has been murdered. The sheriff tells him they do not have the time as there are 30,000 people in town for the Muskie Madness festival and only four sheriff’s deputies. Later, Kiersten is floating on a raft in the lake when a fishing hook is thrown in her direction, but she tries to swim away. Meanwhile, Wayne Duerst catches a forty-six-and-a-half pound fish and is the leader in the Muskie Madness fishing contest. When no one has seen Kiersten for hours, Ann and Peter telephone the sheriff, but the phone lines are dead. Evelyn Duerst comes in talking about how it is a conspiracy. Wayne Duerst arrives and apologizes for shooting Peter’s grandfather all those years ago. Wayne says he offered to pay to have the bullet removed, but Peter’s grandfather refused and instead gave him a job as caretaker at his lodge. That night, Denny Dobyns goes out on the lake in a small power boat, but the engine dies. Denny sounds his horn trying to draw attention, but instead, a fishing hook snags him, pulling him into the water. Later, Evelyn Duerst’s wife Bev goes out for a late night swim in the lake. A fishing hook snags Bev and pulls her under the water. Still later, an intoxicated Finner goes out in a boat on the lake, playing music on his portable radio while the cicadas chirp. In his bait shop, Leroy Leudke is driven mad by the sound of the music and the cicadas. Leudke casts his fishing line toward Finner. The fishing hook hits Finner’s ear, cutting it off. Finner jumps in the water, but Leudke kills him. In the bait shop, he cuts off Finner’s arm, then grinds it up for fishing bait. Kiersten returns from her swim, upset because she found Finner’s empty boat in the water with blood in it. Peter discovers Finner’s cut-off ear in the boat. Meanwhile, Leudke goes to the dock outside his bait shop and pulls four dead bodies out of out of the water, each hooked up to a single rope. Peter, a music major, realizes that the sound of the cicadas chirping combined with the music from a radio creates a frequency pitch known as the “Devil’s tri-tone.” That musical combination was outlawed for a century because it drives people crazy. A young boy says that Leudke has a metal plate in his head from an injury he received during the Korean War. Peter reports the metal plate could amplify the Devil’s tri-tone even more, so he and Evelyn Duerst go to Leudke’s to investigate. There, they find pots full of human organs. Meanwhile, Ann goes to the dock to meditate. Approaching in a row boat, Leudke throws a fishing line which hooks Ann’s hand, but she pulls it off and runs away. Leudke gives chase and catches her, bringing her back to his bait shop. Ann tries to talk Leudke out of killing her, but he ties her up, stuffs her inside an empty refrigerator and closes the door. Wayne Duerst is not convinced Leudke is the mad killer until they show him the special Cooper bullet with which Duerst accidently shot Peter’s grandfather. The next morning, Leudke wins the Muskie Madness contest with a fifty-pound fish. Wayne Duerst accuses Leudke of being the killer, but the sheriff does not believe him. Peter gets Duerst to teach him how to cast as well as his grandfather. That night, Peter takes his boat outside Leudke’s bait shop and turns on his radio. When the cicadas start chirping, Leudke comes out screaming. He casts a line which hooks Peter in the chest. Meanwhile Peter casts a line that hooks Leudke’s neck. As the two struggle, Leudke overpowers Peter. In the bait shack, Leudke sharpens a knife as Peter regains consciousness. Peter turns the radio on, the sound of which drives Leudke mad. Peter rescues Ann from the refrigerator but Leudke comes after them. When he hears sirens approaching, Leudke runs into the woods. Duerst helps the sheriff search for Leudke, whom they presume is dead due to the massive blood loss, while Ann and Peter leave town.