Single American mother Ann Lake (Carol Lynley) arrives in London with her daughter, nicknamed Bunny, to live with her brother Stephen (Keir Dullea). Ann leaves Bunny at a nursery school, but when she returns there is no sign of her daughter and no evidence that she was ever there. Stephen, a journalist, questions all who might have seen Bunny. Once Scotland Yard Superintendent Newhouse (Laurence Olivier) joins in the search, he unearths several disturbing details about Ann.