Gille Jenkins (Hayley Mills), a twelve-year-old tomboy and compulsive liar living in a sordid tenement with her single parent mother, witnesses the murder of an immigrant Polish woman living in an nearby apartment by her former boyfriend, Bronislaus Korchinsky. The young merchant marine becomes distraught when he discovers that she has become the mistress of a married British sports announcer and shoots her with her own gun. Gillie lies about the circumstances in order to keep the gun which she discovers where Korchinsky hid it. She ultimately bonds with him and misleads the police in their investigation.