A band of notorious shipwreckers, led by the Captain [John Rico], force a ship to crash and ransack its contents. The crew includes two subordinates, Le Bosco Willy Braque and Paul Paul Bisciglia, and the Captain's lover, Tina Joëlle Cœur. The ruthless gang brutalizes two young women survivors of the wreck, raping and beating them, and the assault leaves Tina aroused.
While drinking in a local tavern, the Captain begins to have visions of the women's dead bodies. The tavern’s landlady, Louise Louise D'hour, a clairvoyant, suspects that he has a guilty conscience, which drives him into a frenzy. A bar fight erupts as Le Bosco and Paul try to bring the Captain under control. Word spreads that spectres of the two women were seen wandering around a ship cemetery near haunted ruins, where a demon has been imprisoned for a century. The community fears the ruins, and believes the demon's return is imminent.
The shipwreckers believe the women are still alive and track them to the ship cemetery, intent on killing them. They set fire to the ships as a lure, and Tina attacks them before they fight her off and escape. They make their way to the haunted ruins with Tina in pursuit, but even the Captain and the others fear the grounds.
There, the women meet a friendly clown Mireille Dargent and an Exorcist Ben Zimet. The Exorcist informs the demon that the women have arrived and urges mercy if he wants to free him from his confinement. The women stay mute from their trauma, and the Exorcist agrees to aid them in revenge, but only if they release the man imprisoned within the ruins, who is possessed by a demon and unpredictable.
The Captain remains on the cusp of madness, wondering whether the women are alive or dead and fearing that they are coming for him. The demon finally calls the two women to his dungeon; Louise guides them telepathically and helps them unlock the door. The demon makes a pact with the women, giving them his power, and they become the demoniacs of the title.
The women enter the tavern. Le Bosco lunges at them with a blade, stabbing one, though she survives. The shipwreckers flee. Tina taunts the men for their cowardice and heads to the tavern, where she kills Louise. She proceeds to the haunted ruins to attack again, and the women use telekinesis to hurl statues at her. Tina is nearly crushed but escapes. Meanwhile the men seriously wound the clown and the Exorcist. The only way to save them is for the women to relinquish their revenge and stay by their sides, using their powers to keep them alive until dawn, when the powers will fade back to the demon.
The women agree, and the clown and Exorcist are revived. The demon wishes them luck, but warns that both the women and their attackers are fated to die. The next morning, on the beach, the survivors encounter Paul, who is drunk and attacks them with a broken bottle, accidentally cutting his own throat.
The women are then ambushed by the Captain and Le Bosco, who tie them to the remains of a burnt ship and rape them at Tina's behest, while Tina indulges herself. The Captain, overwhelmed with guilt, strangles Tina and stabs Le Bosco. He tries to rescue the women and drown with them, but they all perish together. The Exorcist and the clown watch in despair as they disappear beneath the sea.
