Alan and Kate Marlowe are out an walk with their kids, Davy, Lucy, Sam, and baby Matthew. Kate meets a pregnant stranger and she comes home with them. It is apparent that Alan finds something "off" about her right away, as she intensely stares at him, but he does not say anything. Left briefly unattended, she cuts their telephone line. Alan is about to drive her home, but she goes into labor, and Kate helps her deliver a baby girl. The next day, Kate sees the woman is gone, having abandoned the child with them. Despite Alan's reservations, Kate wants to keep the baby, whom they name Bonnie. Later on, they find Matthew dead in a playpen with Bonnie.
At a family picnic, Davy and Bonnie wander off, and they search for them desperately. Kate finds Bonnie on the bank of a creek with scratches on her hands, while Alan finds that Davy has drowned in the creek. Alan attempts to perform CPR on Davy, but is unsuccessful. Later, Kate and Alan agree that the scratches on Bonnie must have been from Davy saving her. Bonnie starts to break things and Sam gets blamed for them, despite him saying he did not do it. Kate attributes this to Sam's jealousy of Bonnie. One day, the family is playing hide and go seek and Alan finds Sam dead in a barn. Later, Alan finds Bonnie's ribbon next to where Sam's body was.
The Marlowe's begin to receive letters accusing them of killing their children and Kate falls into a depression. When a reporter comes to their house and upsets Kate, Alan agrees to move the family to London. Bonnie becomes ill with the mumps, and purposely kisses Alan as he takes a nap. He becomes ill with the mumps too, and has a flashback in a dream, to the circumstances of the deaths of his sons, and Bonnie being nearby in each one.
At a playground, Alan watches Bonnie throw an unoccupied swing in the path of a swing Lucy is swinging on. The chains on the swing twist together, but Lucy does not fall off, and Alan is able to save her before she is hurt. Alan tries to discuss his concerns about Bonnie with Kate, saying she is not normal. Kate strongly disagrees, saying that Bonnie loves Lucy and was only playing. Alan says Bonnie loves Lucy the same way she loved their three boys, and Kate is disgusted at the insinuation. Alan tells Kate his theories about Bonnie being involved in the deaths, but she is still in disbelief. Alan uses an analogy about Bonnie, saying that a cuckoo lays its eggs in another nest, and the fledgling pushing the others out to get the full attention of the parents.
Alan wants to send Bonnie away, but Kate refuses, so he kidnaps Lucy. Alan goes to see Kate, who is distraught that Alan will not tell her where Lucy is. Alan gives Kate an ultimatum to choose Bonnie or Lucy. She refuses to do so and he leaves. Later, they find out that Kate has had an accident and is in the hospital. Alan rushes back to London, where he learns that Kate had been pregnant, but miscarried due to the accident.
Back at their apartment, Alan finds out from neighbor, Mr. Taverner, that Kate tripped over a doll at the top of a staircase and that Mrs. Taverner has taken Bonnie on a trip. Kate comes Alan's work to tell Alan him she wants a divorce. He is alarmed to learn that Bonnie is home alone with Lucy. Alan calls Lucy, telling her to go next door to the Taverner's. Bonnie has them locked in, and as Kate and Alan get home, Bonnie has used mind control on Lucy to make her jump out of a window to her death. Alan tries kill Bonnie, but Mr. Taverner pulls him off of her. Kate decides to stay with Bonnie, and Alan leaves her. At a park, Alan sees the strange woman who gave birth to Bonnie, and is now pregnant, and talking to the mother. He runs after them to warn the family, but they are already gone.
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