The promise of the divine is but an illusion.
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 to the cult, Thomas arrives and encounters the leader, Malcolm Howe, who founded the cult with two other convicts, Frank and Quinn. They claim that the previously barren island was rendered fertile through blood sacrifice. Each member of the cult keeps a jar for regular bloodletting. Thomas also discovers teenagers Jeremy and Ffion – son of Frank and daughter of Quinn, respectively – sneaking home after a tryst. He forces Jeremy to admit that Jennifer, who Malcolm is presenting to cult members as a captured spy, 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.
Malcolm suspects an infiltrator among the newcomers and questions them. One of the men attempts to assassinate Malcolm, but Thomas intervenes and is badly wounded during the ordeal. Malcolm promptly has him treated and promises that his loyalty will be rewarded. The same night, Malcolm parades Jennifer through the village, claiming she is a traitor and will be killed if her co-conspirator does not come forward. Thomas accesses a tunnel underneath Malcolm’s house and flees from what appears to be an old woman who chases him. He escapes to a beach cave covered in markings, indicating the presence of a deity on the island. 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. Malcolm’s daughter Andrea brings Thomas to a cave and offers to help him. He tells her that he was a Christian missionary who was persecuted in Peking during the Boxer Rebellion for introducing Christianity to China, and has lost his faith as a result. She takes him to a shack in a wheat field to hide.
Ffion and Jeremy meet in her home, where she reveals that she is pregnant and the pair decide to elope. Jeremy briefly leaves and returns to find that Quinn has murdered Ffion in a forced abortion. Jeremy stabs Quinn before fleeing through the field, where guards capture and return him; Thomas and Andrea follow them back. Quinn frames Jeremy for Ffion’s murder and uses a “purification” ritual to murder him with a hand crank in front of the islanders. When Malcolm and Frank return and challenge 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 through the forest and eventually arrive at the barn where The Goddess is being held. Frank enters and is killed by a masked figure, The Grinder, who Thomas witnesses force-feeding Frank’s blood to The Goddess. Sneaking past The Grinder, he finds Jennifer alive but strung up in a sack. As he releases her, he is knocked unconscious. When he awakens, he finds himself tethered to a meat grinding table by hooks embedded into his hands and legs. Thomas escapes and kills The Grinder but not before the device mangles his hand.
Quinn reveals to a captive Jennifer and Andrea that he imprisoned The Goddess after he and Malcolm realized her powers. He tells the women that he will repeatedly impregnate them and use their offspring as blood sacrifices. Meanwhile, The Goddess begs Thomas to set her free – he grants her wish by immolating her. The rest of the village catches fire and the villagers begin to flee to boats. Thomas tracks down Quinn and his captives. The three of them eventually overpower and kill Quinn, but not before Thomas sustains several stab wounds. He collapses, bidding Jennifer and Andrea farewell as they escape by boat, also asking them to pray for him. His faith restored, Thomas is discovered by an injured Malcolm as his blood replenishes the land, the vegetation consuming him.