The Man in the Iron Mask is a 1939 American film very loosely adapted from the last section of the novel The Vicomte de Bragelonne by Alexandre Dumas, père, which is itself based on the French legend of the Man in the Iron Mask.
The film was Peter Cushing's screen debut. It was also notable for being the source of several subsequent remakes. The film was directed by James Whale and stars Louis Hayward as royal twins, Joan Bennett as Princess Maria Theresa, Warren William as d'Artagnan, and Joseph Schildkraut as Nicolas Fouquet.
(Summary from Wikipedia)