The film opens with a dream sequence in which stock market tycoon Paul Marsh discovers a mermaid with razor sharp teeth while scuba diving into a carven pit. Paul awakes on a boat off the shores of Spain, where he is vacationing with his girlfriend, Barbara, and their friends Vicki and Howard. A sudden storm blows their boat against some hidden rocks. Vicki is trapped below deck, and Howard stays with her while Paul and Barbara take a lifeboat to the nearby fishing village of Imboca. During their absence, an unseen creature from the deep attacks the two in the boat.
At shore, Barbara and Paul find no one about, and venture into town until they eventually reach the church, where they find a priest. Barbara convinces him to help them, and the priest speaks with two fishermen at the docks, who volunteer to take either Paul or Barbara to the wreck. Despite Paul's misgivings, Barbara stays to try to find a phone in order to call a doctor while Paul goes to help their friends.
Vicki and Howard are mysteriously missing, however, and Paul is taken back to Imboca, where he is sent to the hotel that Barbara was supposed to have gone to. But she is missing as well, and Paul is left to wait for her in an old, filthy hotel room, where he dreams of the mermaid again. His fitful rest is disturbed by a large gathering of strange, fish-like people approaching the hotel and is forced to flee. He ends up in a macabre tannery full of human skins, where he discovers Howard's remains. He escapes the tannery by starting a fire, and finds momentary safety with an old drunkard named Ezequiel, the last human in Imboca.
Ezequiel explains to Paul about how the village in lean times has fallen to the worship of Dagon. This brought incredible wealth to Imboca in the form of fish and gold, but also horror when Dagon demanded live sacrifices and human women (including Ezequiel's parents) to breed with. Paul begs Ezequiel to help him escape. Ezequiel relents and takes Paul to the Mayor's manor, so he can steal the town's only car. Ezequiel distracts some Imbocans long enough for Paul to slip inside, but he accidentally honks the horn while trying to hot-wire the engine. Forced to flee into the manor, Paul finds a beautiful woman named Uxia, who looks just like the mermaid in his dreams. She saves him from discovery, but when he finds she isn't human either, he flees in horror despite her pleas to stay.
Paul narrowly escapes a horde of villagers in the car, but ends up crashing. He is caught and thrown into a barn, where he is reunited with Vicki, Ezequiel, and Barbara. The three plan to escape, but the attempt comes to naught when they are discovered. Having been raped and impregnated, Vicki kills herself. Paul and Ezequiel are separated from Barbara and end up in a butchery, where they are chained and given a chance to join the worship of Dagon. When they both refuse, Ezequiel is skinned alive before Paul's eyes.
Paul is saved by the appearance of Uxia, who informs Paul that he has no choice but to join them. When he seems to concede, Uxia tells the priest to make arrangements for their marriage. After Uxia leaves, Paul escapes, killing the guards and the priest, and starts looking for Barbara, collecting a can of kerosene on the way. His search brings him to the empty church and a hidden passage that takes him below ground, where a congregation of Imbocans are watching Uxia torture Barbara before she is chained and lowered into a deep, water-filled pit. While the Imbocan congregation and Uxia call to Dagon, Paul attacks, dousing several villagers in kerosene and setting them on fire before pulling Barbara out of the pit. However, the monstrous Dagon himself grabs Barbara and drags her down, devouring her.
The uninjured Imbocans assault Paul, but are halted by Uxia and a monstrously deformed Imbocian who is revealed to be Uxia's father - and Paul's. Uxia explains that Paul's human mother escaped from Imboca years ago, but now that Paul has returned, he will be her lover and they will dwell with Dagon forever. Trapped, Paul pours the last of the kerosene over his own body and sets himself on fire. Uxia grabs him and sends them both into the water, where Paul sprouts gills. With no choice left, he follows Uxia down into Dagon's undersea lair.
Who does the village worship?
Dagon