Thunder Rock is a 1942 British drama film with supernatural elements, directed by Roy Boulting and starring Michael Redgrave, James Mason, Lilli Palmer and Barbara Mullen. The film is based on the 1939 play Thunder Rock by Robert Ardrey, which had originally been a notable stage flop in New York, but proved to be considerably more successful in London where it ran for months in the West End. It was opened out considerably from its source by the addition of a montage sequence to illustrate the protagonist Charleston's back-story, and flashback sequences detailing the histories of the various characters in Charleston's imagination, in the process serving to give a heightened propagandist tone to the material. Critical opinion of the time in Britain was divided as to whether the additional material brought new depths to the story, or made too explicit things which Ardrey had preferred to leave to the audience's imagination and intelligence. The film was however almost universally admired by North American critics and became a huge popular success. Ironically, it ran to packed houses in New York for over three months, where the play had folded in less than three weeks.