In 1905, Thomas Richardson travels to a remote Welsh island to rescue his sister, Jennifer, who has been kidnapped and held for ransom by a mysterious cult. Posing as a convert, Thomas encounters the leader, Malcolm Howe, who founded the cult with two other convicts, Frank and Quinn. They claim that the barren island was rendered fertile through blood sacrifice. Thomas discovers Frank's son Jeremy and Quinn's daughter Ffion sneaking home after a tryst. He forces Jeremy to admit that Jennifer was kidnapped for ransom, as the cult does not have the resources to pay for the continuous animal sacrifices needed to maintain the island's fertility.One of the newcomers attempts to assassinate Malcolm, but Thomas intervenes and is wounded. The same night, Malcolm parades Jennifer through the village, claiming she will be killed if her co-conspirator does not come forward. Thomas flees from an old woman who chases him, and escapes to a beach cave covered in markings, indicating the presence of a deity. Malcolm visits a barn where the old woman who is, in fact, the island's deity, is imprisoned in tree roots; he reprimands her for appearing to Thomas before feeding her his blood, causing the vegetation imprisoning her to bloom. Thomas tells Malcolm's daughter Andrea that he was a Christian missionary who was persecuted in Peking during the Boxer Rebellion for introducing Christianity to China, and lost his faith as a result. She takes him to a shack in a wheat field to hide.Ffion reveals to Jeremy that she is pregnant and the pair decide to elope. However, Quinn murders Ffion in a forced abortion. Jeremy stabs Quinn and guards capture him. Quinn frames Jeremy for Ffion's murder and uses a "purification" ritual to murder him. Quinn calls Malcolm a false prophet and demands that he prove himself by killing Thomas. Frank, enraged by the death of his son, attacks Quinn, allowing Thomas to escape.Frank and Thomas flee and arrive at the barn where The Goddess is being held. Frank enters, intent on killing The Goddess, but is killed by a masked figure, The Grinder, whom Thomas witnesses force-feeding Frank's blood to The Goddess. Sneaking past, he finds Jennifer alive but strung up in a sack. As he releases her, he is knocked unconscious. He finds himself tethered to a meat-grinding table by hooks embedded into his hands and legs. He escapes and kills The Grinder.Quinn reveals to a captive Jennifer and Andrea that he imprisoned The Goddess after he and Malcolm realized her powers. He plans to repeatedly impregnate them and use their offspring as blood sacrifices. The Goddess shows Thomas her history with the cult, and begs him to set her free; he grants her wish by immolating her. The village also catches fire and the villagers flee to boats. Thomas, Andrea, and Jennifer overpower and kill Quinn at the cost of Thomas sustaining stab wounds. He collapses, bidding Jennifer and Andrea farewell as they escape by boat. His faith restored, Thomas is discovered by an injured Malcolm. As he bleeds onto the ground, the vegetation around him grows and infuse within his body, while his eyes turn the same shape and color as the goddess's, signifying his rebirth as the new guardian of the island.