In the picturesque yet troubled town of Noyo, located in the Pacific Northwest, the introduction of a new salmon cannery threatens the livelihoods of many local fishermen. While some, like Hank Slattery, support the project in hopes of increased prosperity, others, including local fisherman Johnny Eagle, resist it fiercely, choosing instead to maintain their independence. Hank, however, harbors a deep-seated racism that he directs at Johnny, intensifying tensions within the community.
As the fishermen go about their business, a horrific discovery awaits them when they unknowingly catch a mysterious monster in their nets. The deep waters claim the young son of one of the fishermen, dragging him into an abyss of terror. A misfire of a flare gun quickly escalates the tragedy, igniting a catastrophic fire that claims the lives of everyone aboard the boat. Witnessing this tragedy are Jim Hill and his wife, Carol, who become increasingly alarmed as the ominous presence of the sea begins haunting their tranquil lives.
Soon after, teenagers Jerry Potter and Peggy Larson venture to the beach, where a night of fun turns into a nightmare when Jerry disappears into the depths. Initially dismissing it as a harmless prank, Peggy’s horror escalates as she discovers Jerry's mutilated remains and finds herself attacked by a lurking monstrous silhouette.
The predator's rampage continues, preying on more unsuspecting youths, leading to a series of brutal assaults. During this turmoil, Hank’s animosity toward Johnny intensifies, culminating in a violent confrontation during a town dance where Dr. Susan Drake tries to address the community as a representative for the cannery. Her scientific observations reveal alarming mutations among local fish populations—mysterious transformations potentially linked to the Canco company's experimental growth hormones.
The deadly creatures terrorizing Noyo are traced back to Canco’s experiments gone awry, culminating in gruesome humanoids birthed from their monstrous heritage. The annual salmon festival turns into chaos as these fiends emerge in force, causing the townsfolk to unite against a common enemy. Jim devises a cunning plan to combat these humanoids by pouring gasoline and crude oil into the water, igniting a fierce battle as townspeople fend off the monsters while they struggle in a world beyond their watery domain.
In a climactic showdown, the citizens of Noyo rally together to fight back against their grotesque adversaries, utilizing every means at their disposal to prevail. In the process, Hank finds himself on the brink of death, only to be rescued by Johnny. Meanwhile, at home, Carol courageously defends herself against another hostile humanoid, ultimately slaying it before it can inflict harm.
As dawn breaks and the dust settles, it appears that normalcy is returning to Noyo. Relationships mend as Jim and Carol reunite, and the townsfolk begin to recover from the devastation. However, in a chilling twist, the narrative culminates in a clinical room where Dr. Drake attends to a traumatized Peggy. The revelation that Peggy is nine months pregnant with a monstrous offspring sends shivers down the spine, as the ominous threat of the humanoids lingers in the shadows, poised to take on a new unimaginable form.
