Description
Neil and Sherry Grice, a young American couple, are visiting Greece in search of Neil's missing sister, Madeline, an artist who traveled to a remote island off the coast shortly before her disappearance. Upon arriving on the island, the couple are met by Nereus, the elderly mayor. While touring the island, Neil and Sherry see Madeline walking along a rocky hill, and follow her. They find her in the company of Frye, a middle-aged treasure hunter who is initially hostile toward Neil.
Later that night, Frye and Madeline's friend, Barbara, use Neil and Sherry's boat to visit a remote area of ocean off the island where Frye scuba dives into an underwater cavern. Inside, Frye gets drunk and uses a homemade bomb to unseal a walled-off tunnel which bears a sigil engraved above it. While Neil and Sherry visit the beach the next day with Madeline, Frye, and Barbara, Sherry observes Madeline acting strangely, in a seemingly trance-like state. When Sherry gifts Madeline an expensive perfume, Madeline pours the entire bottle over her body.
Madeline visits a convent on the island where she speaks with Sister Elena, and confides in a number of bizarre paintings she has uncovered after stripping the paint from the walls of the monastery. Among them are a portrait of a saint and a serpentine creature, which predates Christianity. Later, Barbara is attacked while swimming on the beach and killed in what appears to have been a shark attack. Her dismembered corpse is found by local children after it washes up on the shore. Meanwhile, several other young women in the town go missing.
While out boating, Neil and Sherry come across Frye's boat, and assume he is diving. Neil decides to follow him, and enters the underwater cave. Once inside, Frye tells Neil that it was Madeline's idea to come to the island in the first place, and that he had met her on the mainland prior—she was drawn to the island, and insistent they go there. He recounts how, after they arrived there, Madeline's personality began to change. Neil and Sherry visit the convent in an attempt to locate Madeline, but a nun forces them to leave. Meanwhile, Lethe, a young village girl, falls off a cliffside into the water while playing with the village boys. Her mother attempts to save her, as does Frye, who witnesses the accident from his boat. Fry saves Lethe, but her mother is eaten alive by the serpent.
That night, Frye, Neil, and Sherry attend a festival held by the villagers. Meanwhile, Madeline, back at the convent, witnesses the nuns holding a ceremonial prayer. A short time later, a bloodied and shocked Sister Anna, the mother superior, stumbles into the festival. Neil, Sherry, and Frye rush to the convent and find it in disarray, the nuns all dead. Madeline, meanwhile, in a trance state, jumps from a cliff into the ocean, and swims to the underwater cavern. Frye and Neil follow after her. As Neil helps pull Madeline from the cave, Frye detonates it, blowing up the serpent and sacrificing his own life. Madeline, Neil, and Frye depart the island at dawn.
