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Barbara (Margaret Lockwood) after stealing her best friends fiance and marrying him, she becomes bored with her life in a country mansion and takes to highway robbery as a thrilling way to put some excitement into her life.
One night she meets the dashing Captain Jackson (James Mason), a fellow highwayman, and the two embark on a passionate and nocturnal affair.
Barbara's unusual habit of sleeping all day does not go unnoticed, and when she is recognised as a robber; she turns to murder to keep her secret.