Alan and Kate are a British couple with 4 small children, staying at an isolated country house. A mysterious pregnant lady shows up, and they invite her in to tea. She goes in to labor at their house. After the baby is born the lady disappears. With the mother gone, no clues to find her, and no other claimants, they adopt the baby girl, Bonnie.
Over the next few years, their 3 boys die in suspicious accidents. With the 3rd death, Alan realizes Bonnie is killing the other children. He compares Bonnie to a type of bird that lays it's eggs in other bird's nests. When the fledgling hatches, it throws the real babies out of the nest to get all the nourishment from the parents. Kate refuses to believe it.
Alan moves out with their daughter, Lucy, to keep her safe from Bonnie. Kate goes to his work, to tell him she is divorcing him and keeping both girls. Alan is alarmed to learn that she took Lucy home, and Lucy is alone with Bonnie. Alan and Kate drive back to the apartment. Lucy opens window to wave to them, and they see Bonnie push Lucy out. Lucy falls to her death. Despite watching this, Kate still insists Bonnie is innocent. Kate leaves Alan to devote herself to raising Bonnie.
The movie ends with Alan walking in a park. He sees the same mysterious pregnant lady befriending a young family, across the lake. He tries to run to warn the couple, but by the time he gets there the lady is in their car driving off.
Thanks Joseph C!