An onscreen text warns of the superstitious belief in a ''Vrykolakas'', a malevolent force in human form. The film proper begins during the Greece in the Balkan Wars. While his troops are burying their dead, General Pherides (Karloff) and American reporter Oliver Davis (Marc Cramer) visit the Isle of the Dead to pay their respects to the General's long-dead wife. They discover the crypt despoiled; hearing a woman singing on the supposedly uninhabited island, they set out to find her. They also find retired Swiss archeologist Dr. Aubrecht (Jason Robards, Sr.), his Greek housekeeper Madame Kyra (Helen Thimig), British diplomat Mr. St. Aubyn (Alan Napier) and his pale and sickly wife (Katherine Emery), her youthful Greek companion Thea (Ellen Drew), and English tinsmith Andrew Robbins (Skelton Knaggs).