Cynthia, traumatized by the death of her baby after she left it in a bathtub to drown, decides to go on vacation with five friends: Jeff, Rob, Lynn, Paul, and Terri. They charter a plane in Seattle, but engine trouble forces them to make an emergency landing on a lonely island in the Puget Sound, where they set up a makeshift camp and try to figure out their next move. The next morning, Paul stays behind while the others venture off in search of help, and their wanderings lead them to a large cottage hidden among the trees.
Inside, they meet the elderly couple who live there, the austere Ma Yvonne De Carlo and the stern Pa Rod Steiger, who welcome the group for the night. Over dinner, Ma and Pa reveal a strict, almost ritualistic way of life: Ma frowns upon any smoke, and Pa demands that the party observe clear boundaries to keep the peace. As the evening widens its chill, Lynn and Cynthia discover that Ma and Pa have a child, Fanny Janet Wright, who appears to be a grown woman trapped in the body of a twelve-year-old. The family’s rules tighten around them—no swearing and a rigid separation of the sexes to prevent premarital intimacy.
The following morning brings a grim turn. Rob slips away for a walk and encounters Fanny pushing her brother Woody on a crude swing. Rob agrees to swing himself, only for Woody to climb to the top and sever the rope, sending Rob plummeting to his death on the rocks below. The group mourns the sudden loss, their grief heavy in the air as the island seems to close in around them. Lynn and Cynthia, left outside, whisper about the family’s oddities, a conversation Fanny overhears, and Cynthia, uneasy but trying to be helpful, consoles the frightened girl and reluctantly agrees to participate in more games.
Fanny then reveals her unsettling prized possession—a so-called baby that looks like a doll, but which turns out to be the preserved remains of an infant. Cynthia also meets Teddy William Hootkins, one of the brothers, and Fanny explains that Cynthia is simply her friend. The eerie atmosphere intensifies when Lynn stumbles upon Woody, Teddy, and Fanny playing jump rope in the woods. When Lynn taunts them, the trio retaliates, and she is attacked and presumably killed.
Seeking solace, Cynthia tells Jeff Mark Erickson about the mummified infant, and Jeff tries to comfort her with a kiss. Fanny’s jealousy surfaces as she watches them together, and she rushes to confront the pair, stabbing Jeff in the eye with a knight’s sword and ending his life. Cynthia, terrified and desperate, confesses to Ma, but Ma lashes out, insisting that they are all wicked. Cynthia flees as Ma finishes Jeff off with knitting needles, and she stumbles into the woods—there she finds Lynn’s corpse dangling from a tree and a frightened Terri.
Terri and Cynthia learn that Paul and the plane have vanished, and Woody and Teddy press the girls into a frantic chase through the trees. Terri and Cynthia finally corner Fanny, and Terri draws a gun, forcing Fanny to cooperate to escape the island. They drag Fanny to Ma and Pa, pressing the old couple to help them leave. Pa, in a bid to protect his daughter, finally reveals the location of a small boat. They make their way to a dingy fisherman's craft, where they discover Paul’s corpse with an ax buried in his skull. Cynthia and Terri flee, while Woody and Teddy set Paul’s body ablaze.
Night falls, and Terri and Cynthia seek shelter in a hollow tree, but dawn finds their pursuers again at their heels. They race back to the cottage to grab a radio, only to be swarmed by Woody and Teddy. Teddy chases Terri into the woods, while Fanny toys with Cynthia. Terri is ultimately caught by Teddy, and her neck is snapped in a brief, brutal moment. Teddy rapes Terri’s corpse, an act that Woody announces afterward with a cruel tattletale’s glee, and Teddy bears the physical consequences of his crime in a brutal beating as Cynthia watches in horror.
Cynthia’s resolve hardens in the horror-filled night. She begins to shed her humanity and drift toward the ritual madness of the family, joining in Fanny’s birthday celebration in a pink gown that mirrors the childlike world she’s fallen into. Yet the memories of her drowned baby haunt her at every turn, and the weight of guilt pushes her toward violence. She bludgeons Fanny to death with a metal washbasin, then kills Woody with a sword, and finally stabs Ma with knitting needles. Teddy remains, but Cynthia’s rampage continues when she wounds him with a sickle. Pa returns to a scene of carnage to find his family dead and is shot by his own shotgun by Cynthia, who has embraced vengeance and the ruin of the once-innocent household.
The film closes on a haunting, somber note as Cynthia ascends to Fanny’s room, sits by the cradle, and begins to rock it slowly while singing a soft lullaby, the echoes of the baby’s death and the island’s darkness reverberating through the room. The sanctuary of the cottage has become a mausoleum, and Cynthia, now fully consumed by her trauma, remains beneath the weight of what she has become.
