Anglers from the fishing village of Noyo, California catch what appears to be a monster. The young son of one of the anglers falls into the water and something unseen drags him under the surface. Another angler prepares a flare gun but he slips and accidentally fires it into the deck, which is soaked with gasoline dropped earlier by the boy. The vessel bursts into flames and explodes; everybody aboard is killed. Jim Hill (McClure) and his wife Carol witness the explosion. Later, Jim and Carol’s dog goes missing and the pair finds its dismembered corpse on the nearby beach.
The following day, teenagers Jerry Potter (Meegan King) and Peggy Larson (Lynn Schiller) go for a swim at the beach. Jerry is abruptly pulled under the water. Peggy believes it is a prank until she discovers his mutilated corpse. Peggy screams and tries to reach the beach but a monstrous figure drags her across the sand. The humanoid creature tears off her bikini and rapes her.
That night, two more teenagers are camping on the same beach. Billy (David Strassman) is about to have sex with his girlfriend, Becky (Lisa Glaser) when another humanoid monster claws its way inside, kills him and chases Becky onto the beach. She outruns her assailant but then runs into the arms of yet another monster, which throws her to the sand and rapes her. More attacks follow; not all of them successful, but few witnesses survive to tell the public about the incidents; only Peggy is found alive, though severely traumatized. Jim’s brother is also attacked, prompting Jim to take a personal interest in the matter.
A company called Canco has announced plans to build a huge cannery near Noyo. The murderous, sex-hungry mutations are apparently the result of Canco’s experiments with a growth hormone they had earlier administered to salmon. The salmon escaped from Canco’s laboratory into the ocean during a storm and were eaten by large fish that then mutated into the brutal, depraved humanoids that have begun to terrorize the village.
By the time Jim and Canco scientist Dr. Susan Drake (Turkel) have deduced what is occurring, the village’s annual festival has begun. At the festival, many humanoids appear, murdering the men and raping every woman they can grab. Jim devises a plan to stop the humanoids by pumping gasoline into the bay and setting it on fire, cutting off the humanoids’ way of retreat. Meanwhile, Carol is attacked at home by two of the creatures, but manages to kill them before Jim arrives.
The morning after the festival, normality seems to have returned to the village. Jim asks the sheriff about Dr. Drake. The sheriff mumbles that she went back to the lab, where she is coaching a pregnant Peggy, who has survived her sexual assault. Peggy is about to give birth when her monstrous offspring bursts from her womb, with Peggy screaming at the screeching baby