Paul Marsh dreams of encountering a
mermaid with razor-sharp teeth while exploring an underwater cave. He awakens 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 the rocks, pinning Vicki against the bed and flooding the boat. While Howard stays with her, Paul and Barbara take the lifeboat to the nearby fishing town of Imboca. During their absence, an unseen water creature attacks Howard and Vicki. Barbara and Paul venture through the seemingly deserted town and reach a church. There, they convince a priest to help them. Two fishermen at the docks volunteer to take one of them back to the boat. While Paul goes with the fishermen, Barbara stays to find a phone and call for help. After failing to find either Howard or Vicki, Paul goes to the hotel where Barbara supposedly stays, but she isn't there. He waits for her in an old, filthy hotel room, where he dreams of the mermaid again. Awakened by a large gathering of
strange, fish-like people approaching the hotel, he is forced to flee. Hiding inside a
tannery, Paul discovers Howard's skinned corpse. He escapes the tannery by starting a fire and finds momentary safety with an old drunkard named Ezequiel, the last full-blooded human in Imboca. Ezequiel explains that, many years ago, the village fell on lean, hard times. Captain Orpheus Cambarro (based on captain Obed Marsh) convinced the locals to worship Dagon rather than the
Christian God. Orpheus's first offerings to Dagon brought Imboca enormous wealth in the form of fish and
gold. This caused the locals to make him the high priest of the Church of Dagon, helping him dismantle the local
Catholic church and killing the priest to establish the church in Dagon's honor (based on the Esoteric Order of Dagon). However, Dagon soon demanded
blood sacrifices and human women to breed with, which the villagers and Cambarro blindly complied with. Ezequiel's father and mother, who resisted Orpheus' heretical practice, ended up as sacrifices. Over time, the people of Imboca began to die off, leaving only the half-fish descendants of Dagon. They kidnap unsuspecting visitors to either breed or sacrifice to Dagon. Ezequiel could only watch the village go to ruin and lament the villagers' foolishness in worshiping a demon for short-lived prosperity. Paul begs Ezequiel to help him escape. Ezequiel takes Paul to the Mayor's manor, so he can steal the town's only car belonging to Xavier Cambarro, Orpheus's grandson. Paul sneaks into the car and tries to hot-wire it, but accidentally honks the horn. Cornered, he flees into the manor and finds a beautiful woman named Uxia, the mermaid from his dreams. She saves him and urges him to stay, but when he finds that instead of legs Uxia has tentacles, he flees in horror. Paul successfully starts the car and drives away, but ends up crashing. Paul is captured and thrown into a barn, where he is reunited with Vicki, Ezequiel, and Barbara. The three plan to escape, but the attempt is foiled. The traumatized Vicki, having been impregnated by Dagon, kills herself. Paul and Ezequiel are chained up in a butchery and offered the last chance to worship Dagon, which they refuse. Paul apologizes to Ezequiel, who thanks Paul for helping him to remember his parents. Paul helplessly watches as the cult flays Ezequiel alive while the captives recite the
23rd Psalm together. Uxia talks down the cult and saves Paul. He offers to stay in exchange for Barbara's release, but Uxia insists that Barbara must bear Dagon's child. When Paul seems to concede, Uxia tells the priest of Dagon to make arrangements for their marriage. After Uxia leaves, Paul kills the guards and the priest and escapes. When Paul reaches the church, he discovers a hidden passage leading to an underground ritual chamber. As a congregation of Imbocans watches, Uxia offers a naked Barbara - who is wearing golden ceremonial jewelry - handcuffed and shackled to a trapeze to Dagon, lowering her into a deep pit leading to the sea. Paul sets several cultists on fire and winches Barbara back up. Barbara, having lost her mind to Dagon, begs him to let her go because she is pregnant. Paul refuses and he tries to untie Barbara, but he doesn't have the key to her handcuffs. Two large tentacles emerge from the water in the pit below and drag Barbara down into the water, leaving only her severed hands behind on the trapeze. The uninjured Imbocans attack Paul and attempt to beat him to death, but are halted by Uxia and a monstrously deformed Imbocan, who is revealed to be Xavier Cambarro; Uxia and Paul's father. Uxia explains that Paul's human mother escaped from Imboca years ago after being impregnated by Xavier, but now that Paul has returned, he will be her lover and they will dwell with Dagon and Barbara, who has now become his newly married bride. Trapped and shocked that he has been an abomination all along, Paul attempts to set himself on fire. Uxia grabs him and dives into the water, at which point, Paul sprouts gills on his chest. With no other option left, Paul embraces his fate and follows Uxia down into Dagon's undersea lair. ==Cast==