Teenager Johnny Hanson Ron Howard arrives at a remote New England fishing village and makes his way to Ronda's Cafe, where he takes a seat at a quiet table. When the owner Ronda Carlson Cloris Leachman comes over to take his order, he asks her name, and after she answers, he departs. Shaken, Ronda returns to the kitchen where Eddie Martin Bobby Darin, the cook and her boyfriend, asks if that was him, and she nods.
Johnny then walks to Cara's house, where he notices Cara's teenage daughter Celia Tessa Dahl watching him from the street and smiles. Noticing her daughter's interest in the stranger, Cara scolds Celia, then yells at her son Porgie. Roy, the town's police chief Simon Oakland, pulls Johnny over and takes him to the station to question him. Frisking him, Roy discovers several letters and explains that several people have disappeared recently; he needs to question strangers. Reading the letters, Roy learns that Johnny is the son Ronda gave away for adoption years earlier. Johnny says he has come to town to learn who his father was, but Roy warns him to leave immediately.
On release, Johnny passes Cara's house again; Celia lures him inside, then spies on neighbor Piccolo Joseph Mascolo with Crystal Kathie Browne, Piccolo's mistress. Celia tries to seduce Johnny, but he declines. Johnny tells Celia he learned that Ronda was his mother from letters she had sent to Johnny's foster parents. Celia explains that Cara is Ronda's estranged sister and leads Johnny into her late father's office to show photos of their father, dead before Celia was born. When Cara returns, Celia locks Johnny in the office, but he escapes through a window.
At the cafe, Eddie demands to know if Ronda intends to let Johnny live there. When Ronda answers 'yes', Eddie, furious, announces he is quitting and leaving Ronda. Finding Johnny outside, Eddie beats him up. Hearing noises, Ronda hurries outside and, finding the bloodied Johnny, helps him into her house. Johnny sees that Ronda has tacked pictures of him onto the wall, and Ronda confesses regret over giving him away. Johnny demands to know who his father was, and when Ronda refuses to tell him, he leaves.
After church that Sunday, Cara's neighbor Florence invites Johnny to her house. Florence reveals that Cara and Ronda have not spoken since Cara's husband George was murdered in his front yard.
Later, Porgie visits his sympathetic aunt Ronda, who reveals that Johnny is her son and asks him to pass the information to Cara. Porgie, continually disparaged by his mother, suggests that Ronda tell her herself. Celia, meanwhile, has continued to spy on Piccolo and Crystal, and throws him a paper airplane containing a message to meet her at the summerhouse.
Crystal, amused, insists that Piccolo attend and accompanies him there, waiting outside. When Piccolo fails to reappear, Crystal enters and finds Piccolo's corpse. Panicked, Crystal tries to escape, but the door is locked. After discovering another dead body in the bathtub, Crystal turns to see Celia holding a cleaver. Terrified, Crystal jumps out a window to her death.
As Porgie is fishing, his net pulls up a corpse. That night, Johnny visits Ronda again and demands to know his father's name. As Johnny flips through old photo albums, Ronda informs him that his father is dead. Johnny finds a clipping about his father and runs to Cara's house, telling Cara that he now knows her husband George was his father.
Returning home, Celia takes a knife from the kitchen. Cara admits that when she discovered George had an affair with her sister, she stabbed her stomach with a knife to abort the fetus; the baby Celia survived but was born insane. Cara then admits to killing George in a jealous rage. Offering to give Johnny George's watches and rings, Cara goes upstairs for them. As Johnny waits in the living room, he hears Cara scream, and sees Celia running down the stairs. Rushing upstairs, Johnny finds Cara dead. When Johnny walks out the door, Celia tries to stab him and they struggle for possession of the knife...
Next morning, Roy takes Celia to jail as Johnny hitches a ride out of town.
