In the Pacific Northwestern town of Noyo, many fisherman have their livelihood endangered due to a new salmon cannery being built which many of the townspeople support. Hank Slattery (Vic Morrow) and several others are for the cannery because it will make life more prosperous for them. Slattery is a racist and takes out his biased aggression on a local fisherman Johnny Eagle (Anthony Penya) who is against the cannery and wants to remain an independent businessman.
In a fishing boat, a group of fisherman catch what appears to be some kind of monster in the netting of their boat. The young son of one of the fishermen falls into the water and is dragged under the surface by something unseen. Another fisherman prepares a flare gun, but he slips and shoots it accidentally into the deck, which is soaked with gasoline dropped earlier by the boy, causing the vessel to burst into flames and then explode; everybody onboard is killed. The explosion is witnessed by Jim Hill (Doug McClure) and his wife Carol, who begin to wonder what is going on here, especially when Carol's dog goes missing during the following night and the two of them find its dismembered corpse on the nearby beach.
The following night, teenagers Jerry Potter (Meegan King) and Peggy Larson (Lynn Schiller) go for a swim at the beach. Jerry is abruptly pulled under. Peggy believes it is simply a prank until she discovers his horribly mutilated corpse. The screaming girl tries to make it to the beach but she is attacked and dragged onto the sand by a monstrous figure. The humanoid thing tears off her swimming suit and rapes her.
The following night on the same beach, two more teens are about to have sex in a small tent when another humanoid monster claws its way inside. It brutally kills the boy and chases the girl onto the beach. She manages to outrun her assailant but then runs straight into the arms of yet another humanoid. The monster throws her onto the sand and rapes her.
Meanwhile, Jim becomes concerned because many other dogs in the town were killed the same way, except Johnny Eagle's dog. Hank Slattery's dog was killed as well and he presumes that Johnny Eagle is responsible and kills his dog, an action that causes a fist fight at one of the local dances where Dr. Susan Drake (Ann Turkel) is being introduced to the community as a spokesperson for the new cannery to be built in town. Drake has come to conduct a scientific study of the local fish life and notices an unusual amount of chemicals has created mutations, large fish-like creatures that actually are in the process of mating with the female population of humans in the town.
A company called Canco has announced plans to build a huge cannery near Noyo. It turns out that the murderous, sex-hungry mutations are apparently the result of Canco's experimentation with a growth hormone they had earlier administered to salmon. The salmon escaped from the laboratory facilities into the ocean during a storm, and were then eaten by other, larger fish who proceeded to mutate into the brutally depraved humanoids who have begun to terrorize the village.
At the climax, the humanoids show up in droves, relentlessly murdering the men and raping every woman they can grab at the annual salmon fest carnival. As a result, many of the conflicts that divide the people are overcome in the effort to band together to fight the monsters. Luckily, Jim devises a plan to stop the marauding beasts. He and Dr. Drake take a boat out onto the water of the marina and pour gasoline and crude oil into the water to burn the humanoids that are in the water. The rest of them crawl out of the water where they cannot survive for more then 10 minutes out of the water. The mob of townsfolk then decend upon the humanoids as they grow weaker the more they are out of the water, and the people shoot, beat, or burn them all to death. Hank is wounded and nearly killed by a Humanoid, but is saved by Johnny Eagle.
At Jim's house, his wife Carol is attacked by a humanoid that breaks in, but she sucessfully defends herself and hacks the creature to death before it can violate her.
The morning after the carnival, everything seems about to return to normal as the creatures are all dead, and the town begins to clean up the damage. Jim and Carol are safely reunited. However in the final scene, Dr. Drake is in a OB-GYN room at a local hospital with a scarred Peggy who has survived her sexual assault is apparently nine months pregnant. Peggy is about to give birth when her monstrous humanoid offspring suddenly bursts out of her stomach in a gory fountain of blood...