Marianne (Yvonne Monlaur), a beautiful young French girl, is stranded en route to a teaching assignment in Eastern Europe. She is persuaded to spend the night at the nearly deserted castle of a mysterious Baroness (Martita Hunt). In the castle, Marianne accidentally discovers a man chained to the wall of his room. The Baroness only explains that he is her "sick and feeble-minded" son. Unable to get any further information from the maid Greta (Freda Jackson), Marianne steals a key and sets him free. Once unbound, the Baron (David Peel) fiendishly recruits the undead for his evil purposes until captured by Marianne and the indefatigable Dr. Helsing (Peter Cushing) in this spooky Hammer Films chiller.