Alan Marlowe and his wife Kate Marlowe are out for a walk with their kids—Davy, Lucy, Sam, and their baby Bonnie. A pregnant stranger, The Stranger, catches their attention, and she is invited into their home. From the moment she arrives, there’s something uneasy about her—the way she stares at [Alan Marlowe], a moment that doesn’t go unnoticed by him. When left briefly alone, she cuts their telephone line, and what begins as a tense intrusion quickly turns into something far more unsettling: she goes into labor, and Kate assists in delivering a baby girl. The next day, the woman has vanished, leaving behind the infant they’ve quietly started to care for, Bonnie, whom they decide to raise despite Alan’s misgivings.
The household is thrown into tragedy as Matthew, their own baby, is found dead in a playpen—an eerie coincidence that seems to shadow Bonnie’s arrival. At a family picnic, [Davy] and Bonnie wander off, and a desperate search ensues. Kate discovers Bonnie along a creek bank, hands scratched, while Alan discovers that [Davy] has drowned. An exhausting attempt at CPR proves futile for Davy. The family grapples with the notion that the scratches on Bonnie might be the price of Davy’s life, and the atmosphere fills with suspicion and grief. As time passes, Bonnie’s behavior grows increasingly destabilizing: she begins to break things, and [Sam] becomes the scapegoat in the eyes of others, even though he insists he didn’t do the damage. Kate finds herself torn between protection and fear, interpreting Sam’s alleged jealousy as a child’s response to their new sister.
During a hide-and-seek game, a chilling incident unfolds: [Sam] is found dead in a barn, and later Bonnie’s ribbon is found near the site where Sam died, deepening the sense that Bonnie is somehow connected to the family’s misfortunes. The Marlowes start receiving anonymous letters accusing them of harming their children, pushing Kate into a deepening depression. A reporter’s visit to the house unsettles Kate further, prompting Alan to propose moving the family to London in search of a safer, steadier life. Bonnie falls ill with the mumps, and she purposefully kisses Alan while he is resting, triggering his own illness and triggering a dream-like flashback to the earlier deaths and Bonnie’s involvement in each instance.
In a playground scene, Alan narrowly saves [Lucy] from harm when Bonnie, perhaps testing her power, sends an unoccupied swing toward Lucy’s path. The chains twist, but Lucy escapes unharmed, and Alan expresses his growing worry to Kate that Bonnie isn’t normal. Kate remains in denial, insisting Bonnie’s affection for Lucy mirrors her love for the three boys, even as Alan’s suspicions deepen. He articulates a theory about Bonnie—the notion of a sparking “cuckoo in the nest” that could be pushing others aside for Bonnie’s own attention—and pleads with Kate to consider sending Bonnie away. When Kate refuses, Alan absconds with Lucy, presenting Kate with a grim ultimatum: Bonnie or Lucy. She refuses to choose, and he leaves.
A wrenching turn follows: Kate is hospitalized after an accident, and Alan rushes back to London only to learn that Kate had been pregnant but miscarried due to the crash. Back at home, a neighbor, Mr. Taverner, reveals that Mrs. Taverner had taken Bonnie on a trip after Kate’s fall. Kate confronts Alan at his workplace to demand a divorce. The tension peaks when Alan discovers Bonnie is left home alone with Lucy. He calls Lucy to move next door to the Taverners, while Bonnie locks them inside and then uses some unseen control to coax Lucy into leaping from a window to her death. Alan fights Bonnie, but Mr. Taverner intervenes, pulling him away. Kate, faced with incredible pain and fear, decides to stay with Bonnie, while Alan walks away from the marriage and the house he once shared with them.
In the final moments, the reflective, anxious tone returns as Alan sits in a park and glimpses the pregnant stranger once more. The woman is accompanied by the mother who welcomed Bonnie into the world, a scene that hints at a disturbing cycle continuing beyond the family’s door. Alan chases after them, trying to warn the family circle, but the mysterious trio has already slipped away, leaving behind a haunting sense that some forces, and some fates, are larger than a single family’s attempt to live normal life.
