This film is based on a 1957 novel by an American writer, Evelyn Piper (a pseudonym for Merryam Modell). It is set in the United States. Otto Preminger bought the film rights during the 1950s, but was dissatisfied with the novel's plot, especially the ending. At one point, he hired the playwright and novelist Ira Levin to write a screenplay, but eventually came to the conclusion that a change of locale would be for the best and hired the then-married writers John and Penelope Mortimer to rewrite the story, with a London setting which would emphasise the aloneness of the heroine. They also provided a new explanation for the child's disappearance and a new villain (with an alarming motive).