One night, a boat reaches Snape Island, and two seamen step ashore to investigate. They uncover a severed hand on a male corpse and push toward a lighthouse, where a second severed body—the head—is found. The men split up to search, and soon another murder surfaces. A young woman, Penny, Candace Glendenning, hides in a closet. Traumatized, she fatally stabs the older seaman, while the boat's captain survives the night.
At the hospital, Penny is examined by doctors. She is shocked, but gradually speaks and recalls how her friends arrived, her memories flashing back in vivid, terrifying fragments. The experience leaves her shaken, and her recollections erupt in a scream as the night’s terrors come rushing back.
Following Penny’s return to health, scientists decide to venture to Snape Island, drawn by rumors of gold and Phoenician treasure. The team—a group of five men and two women—embarks on the Sea Ghost, with the surviving seaman at the helm. The mission is led by Dr. Simpson [Anthony Valentine], and the group carries a mix of curiosity, fear, and resolve.
Upon landing, the crew notices an eerie lack of life: no seabirds, no usual island sounds, and a palpable stillness that suggests a hidden inhabitant. Their leader, Dr. Simpson, surveys a stash in a mansion and starts piecing together the larger story. The captain spots a photograph of his brother in the mansion and reveals a troubling family history: his brother, Saul, along with his sister-in-law and their child, died on the island, though Saul himself appears to linger in the caves beneath the house.
As the group moves deeper, an uncanny noise unsettles them, and the scientists split to search for its source. While two of them briefly engage in sex, the others pursue the Doctor to the boat’s location, only to discover the radio has been blown up—cut off from outside help. The Doctor, who has a long history with the island, has been stalking Saul all night and even admits that he fired on Saul at some point, though the truth remains tangled in their perilous texture of loyalties and fear.
Nora Winthrop [Anna Palk] becomes a focal point as danger closes in. She and Rose Mason [Jill Haworth] share a tense moment inside the house, while Nora is stalked by the bearded Madman amidst the eerie, decomposing images surrounding the estate. Nora encounters a rotting body in a rocker, screams, and collapses. The others rush to her side and press the captain for answers about the dead, the living, and the secrets buried on the island. The captain confesses that the corpse Nora saw is his sister-in-law, and that Saul remains alive, living in the caves beneath the house. The killer’s radio silence deepens the group’s fear, and the search for Saul becomes a relentless chase.
In the caves, Rose hears weeping and, drawn by the sound, confronts Saul as he cradles the corpse of his dead wife. He stalks Rose, and when Nora runs to the lighthouse, Saul throws her from the balcony onto the rocks below. With Nora dead, Rose and the remaining men decide to press into the cave together to find Saul. Meanwhile, the Doctor, who has lingered near a coffin, uncovers the island’s treasure and confirms that his pursuit of Saul has occupied his night as well.
As the group reconvenes, the Doctor explains that he has been on the island before and that he left a boat and supplies on the far side. The captain stays with Rose, while the others retrieve the boat and some artifacts. They head back toward the lighthouse, where the captain reveals that Martha never wanted Saul to be reported. The captain climbs upstairs to rest, only to return bleeding. He speaks of Michael, Saul’s son, and dies soon after. The deformed Michael attacks Rose, who flings an oil lamp at him, setting him ablaze. The house and the connected cave ignite in the flames, and Rose escapes as the two surviving men watch the burning dwellings go up around them.
In the end, the island’s terrible history consumes the living and leaves the survivors to reckon with treasure, vengeance, and the price of secrets kept in the shadowed depths of Snape Island. The ordeal is told through a taut, claustrophobic tension that blends mystery, horror, and a stark, fatal resilience in the face of an inexorable doom.
