In 1905, Thomas Richardson embarks on a perilous journey to a secluded Welsh island, driven by the desperate need to rescue his sister, Jennifer, who has been abducted by a shadowy cult. Posing as a recent convert, he soon encounters the enigmatic leader, Malcolm Howe, who established the cult alongside two other convicts, Frank and Quinn. Their disturbing claim states that the island, though barren, has become fertile through sinister blood sacrifices.

During his investigation, Thomas discovers Frank’s son, Jeremy, and Quinn’s daughter, Ffion, sneaking back from a secret rendezvous. Through a tense confrontation, he compels Jeremy to confess that Jennifer’s kidnapping was a ransom scheme orchestrated by the cult, which is struggling to provide the constant animal sacrifices necessary to sustain the island’s eerie fertility.

Chaos soon unfolds when a newcomer attempts to assassinate Malcolm; however, Thomas intervenes and is wounded in the process. Later that same evening, Malcolm parades Jennifer through the village, ominously proclaiming that she will die unless her accomplice reveals himself. In a frightening encounter, Thomas narrowly escapes from an ancient woman who is somehow connected to the island, fleeing to a mysterious cave covered in cryptic markings that hint at a hidden deity.

Malcolm later confronts the real Goddess, who is entwined in tree roots within a barn, chastising her for her interaction with Thomas before he feeds her his blood. This act inexplicably causes the vegetation binding her to flourish. As Thomas reveals to Andrea, Malcolm’s daughter, that he was once a missionary shunned in Peking during the Boxer Rebellion for his efforts to spread Christianity in China, he confesses that he has since lost his faith.

Amidst the turmoil, Ffion reveals to Jeremy her pregnancy, igniting their plans to elope. Tragically, Quinn mercilessly kills Ffion during a forced abortion, leading Jeremy to stab Quinn in a fit of rage. Jeremy’s fate takes a darker turn as Quinn deceitfully frames him for Ffion’s murder, intending to execute him through a gruesome “purification” ritual. In a bitter confrontation, Quinn openly challenges Malcolm’s authority, branding him a false prophet, demanding he eliminate Thomas in order to prove his worth.

In the midst of this strife, the grief-stricken Frank retaliates against Quinn for the loss of his son, providing Thomas an opportunity for escape. Together, Frank and Thomas venture to the barn where the Goddess is confined. However, their hopes are dashed when Frank is slain by a masked figure known as The Grinder, who malevolently force-feeds Frank’s blood to the Goddess. In a daring maneuver, Thomas rescues Jennifer, only to be knocked unconscious in his efforts.

Upon regaining his senses, Thomas finds himself immobilized on a meat-grinding table, with hooks painfully embedded in his skin. After a harrowing escape where he subsequently kills The Grinder, he learns from Quinn about his twisted plan to exploit both Jennifer and Andrea for future sacrifices, keeping the Goddess imprisoned for his vile purposes. The Goddess reveals her tragic history with the cult and pleads for freedom, which Thomas grants in a heart-wrenching sacrifice when he immolates her.

In a frenzy of chaos, the village ignites in flames as the cultists attempt to flee towards their boats. In a heroic stand, Thomas, alongside Andrea and Jennifer, confronts and defeats Quinn, though Thomas sustains grievous injuries in the process. As he collapses, he bids a sad farewell to Jennifer and Andrea, who escape by boat. Thomas’s faith reinvigorated, he is soon found by a wounded Malcolm. In a surreal twist, his blood spills onto the soil, causing the surrounding flora to flourish and intertwine with his being, symbolizing his transformation into the island’s new guardian, with his eyes reflecting the divine essence of the Goddess he has so fiercely fought to liberate.