A pair of German tourists visiting a remote Greek island go to the beach, and are slaughtered by someone who emerges from the ocean. The young woman is dragged underwater and killed off-camera. The unseen person emerges from the water and kills the man on the beach by bashing his head with a meat cleaver.
Some time later in Athens on the Greek mainland, five travelers, the very pregnant Maggie (Vanessa Steiger), her husband Arnold, his friend Andy, another friend named Daniel, and Carol (Zora Kerova), Daniel's girlfriend, are preparing to tour the islands, and are joined by Julie (Tisa Farrow), who asks for a ride to an island that some friends of hers live on. The only one who objects to this detour to the island (which Julie explains has only a few permanent residents, and only sees tourists a few months out of the year) is Carol, whose tarot cards convince her that something bad will happen if they go to the island. The group sails to the island anyway, and while disembarking the pregnant Maggie hurts her ankle, so she stays behind on the boat with its owner. An unseen man attacks the boat, ripping the sailor's head off, and abducting Maggie.
The others explore the island's town, discovering it in disarray, and abandoned with the exception of an elusive woman in black, who writes "Go Away" on a dusty window. In a house, a rotting corpse which appears to have been cannibalized is uncovered, prompting everyone to rush back to the boat, which is adrift. With no other options, the group goes to the house owned by Julie's friends to spend the night, where they find the family's blind teenage daughter, Henriette (Margaret Donnelly) hiding in a wine barrel in the basement. After wounding Daniel in a panic, Henriette is calmed down, and rants about there being a madman who smells of blood prowling the island.
To stop Daniel's wound from becoming infected, Andy and Arnold go into the town to search for antibiotics. Carol walks in on Daniel flirting with Julie, and goes into hysterics, running off into the night. Julie goes after Carol, but loses her, and meets up with Andy and Arnold. Back at the house, the disfigured killer (George Eastman) breaks in and rips Daniel's throat out, but leaves Henriette alone and flees as the others return.
In the morning, everyone treks through the island, and find a mansion belonging to Klaus Wortman. Julie mentions that she read that Klaus, his wife, and their child are assumed dead, having been shipwrecked, a tragedy which caused Klaus' sister Ruth to become unhinged. Ruth (the woman in black from earlier) watches the group enter the building, comforts the sleeping Carol, and hangs herself.
After waking Carol, Andy and Arnold look out a window, and see that the boat has drifted close to shore. The two men go to secure the vessel, and Julie finds a partially destroyed journal among the objects in the mansion, and it reveals that the killer is Ruth's brother, Klaus, and that the bodies of all of Klaus' victims are in a hidden room.
Andy and Arnold split up, and while Andy tries to swim out to the boat, Andrew reaches an abandoned church and ventures into the underground catacombs, where he finds Maggie, and is confronted by the disfigured Klaus. Klaus has a flashback that reveals he and his family were stranded in a raft after being shipwrecked, and that Klaus accidentally stabbed his wife while trying to convince her that they should eat the body of their dead son to survive. Klaus then ate his wife and son's corpses, driving him insane. Back in the present, Klaus regains his composure, fatally stabs Arnold, strangles Maggie and rips out and eats her unborn child.
At the mansion, Julie uncovers the room where Klaus' victims are, and skims another diary she finds in it. Carol stumbles into the chamber, and drops dead from a slit throat. Klaus then attacks Julie, who locks herself and Henriette in the attic after a short chase. However, Klaus doubles back and breaks through the ceiling and kills Henriette by biting and ripping her face off. Klaus is wounded and then knocked off the roof by Julie when she arms herself with a pickax and stabs Klaus in his leg. The disfigured killer falls off the roof and into a well.
Armed with the pickax, Julie ventures downstairs and peers down the well, and Klaus attacks her. However, she gets her left wrist entangled with the rope used for the bucket of well water while Klaus falls back into the . She climbs back up a the well ladder with Klaus in pursuit, but she still has her wrist entangled in the well rope. Just when Klaus emerges from the well and pulls Julie by the rope to bring her close to him, she is saved at the last minute when Andy appears and stabs Klaus in the stomach with the pickax left on the ground beside the well, causing the cannibal's intestines to spill out. As a last dying act, Klaus gnaws on his own innards, staring at Andy, while Julie looks at Klaus in horror. Klaus then falls over and dies. The final shot shows Andy and Julie standing over the dead killer, staring at each other.