Screen legends Charles Laughton and Boris Karloff haunt the dungeons of a medieval chateau in the Strange Door, a horrific adaptation of the classic Robert Louis Stevenson story. The Sire de Maletroit (Laughton), is an evil French nobleman so obsessed with hatred for his own brother (Paul Cavanagh) that he imprisons him in the castle dungeon. The Sire tries to destroy the life of Cavanagh's daughter (Sally Forrest) by forcing her to marry rogue (Richard Stapley), but plans are upset when they fall in love. Aided by Voltan (Karloff), an abused servant, the lovers attempt to escape but the Sire imprisons them in a cell, whose closing walls may mean violent death for these innocent victims.