Cynthia is traumatized by the death of her baby after leaving him in a bathtub, where he accidentally drowned. She and five of her friends, Jeff, Rob, Lynn, Paul and Terri decide to go on a vacation. They take a plane somewhere, but are plagued by engine troubles and are forced to land the plane on a lonely deserted island. The six set camp and the next morning, Paul stays at the camp while the others set off to find help. They come upon a large cottage nestled in the woods.
After entering the cottage and fooling around a bit, they meet the owners, an elderly married couple going by the simple names of Ma and Pa. The group of friends are welcomed to spend the night. At dinner, Lynn starts smoking and Pa scolds her and demands that she smoke outside. Later, Lynn and Cynthia discover Ma and Pa have a child, Fanny, who looks more like a middle-aged woman but claims she is 12. That night, Ma and Pa throw more strict rules in, such as no cussing and forcing the girls and boys apart to prevent premarital sex.
The night passes by and the next morning, Rob goes for a walk and finds Fanny pushing her brother, Woody, on a crudely crafted swing. Rob is invited to swing and agrees, only for Woody to climb to the top and chop the rope, sending Rob down the rocky cliff below to his death. The group finds out about Rob's death and mourn his loss. Later, Lynn and Cynthia are outside and Lynn talks about how this family is a bunch of freaks and Fanny overhears. Cynthia sees this after Lynn leaves and consoles Fanny and reluctantly agrees to play games with Fanny. Fanny shows Cynthia her baby, thought to be just a doll, but it turns out to be the remains of an infant. Cynthia meets another brother, Teddy, and Fanny explains to him that Cynthia is her friend. Meanwhile, Lynn stumbles upon Woody, Teddy, and Fanny playing jump rope in the woods. After Lynn insults them, she is attacked and presumably killed by the three.
Cynthia tells Jeff about the mummified infant. Jeff tries to console Cynthia and they kiss. Fanny, who wants Jeff to herself, sees and becomes jealous. She confronts the two and kills Jeff by stabbing him in the eye with a sword on a knight statue. Cynthia explains her fear to Ma, but Ma attacks her saying they are all wicked people. Cynthia flees and Ma finishes Jeff off with her sewing needles. Cynthia runs into the woods, finding Lynn's corpse hanging from a tree and a frightened Terri. Cynthia explains everything to Terri and learns that Paul and the plane are missing. Woody and Teddy find the girls and chase them into the woods. Terri and Cynthia find Fanny, and Terri reveals a gun and holds Fanny at gunpoint, forcing her to help them get off the island. They show Fanny to Ma and Pa and demand their help to get off the island. To save his daughter, Pa tells the girls where a boat is. He leads them to a dinky fisherman's boat, where Paul's corpse is lying, an axe buried in his skull. Cynthia and Terri flee and Teddy and Woody light Paul's body on fire.
Night comes and Terri and Cynthia spend the night in a hollow tree. Terri and Cynthia run back to the cottage to find a radio, but are attacked by Woody and Teddy. Teddy pursues Terri into the woods and Fanny toys with Cynthia. Terri is caught by Teddy and has her neck snapped. Teddy rapes Terri's corpse, Woody tattletales and Teddy gets a beating as Cynthia watches in horror. Cynthia eventually breaks down and becomes one of the family, celebrating Fanny's birthday and dressing in a pink gown to match Fanny. However, the flashbacks of her baby drowning get to her; she goes mad, bludgeoning Fanny to death with a metal washtub. She then murders Woody with the sword. She then confronts Ma and stabs her to death with her sewing needles. She finds Teddy and stabs him with a sickle. Pa discovers his family dead and goes outside only to be shot by his own shotgun by Cynthia who has taken revenge and killed the demented family. The film ends as Cynthia goes upstairs, sits in Fanny's room and slowly begins rocking the cradle, singing a soft lullaby.
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