In the mountains of Roumania, professor Blander and his guide Mogar discover an old church and therein an enormous coffin sealed with 13 locks. For two weeks the professor deciphers 13 stone slabs. Each time one of the locks snaps open. In the end they will have freed the devil in person.
Dennis Langley tries to drive his foster-mother insane to get his hands on the family money. He impersonates her dead husband Jeffrey who used to whistle an Italian opera aria. Langley brings in his crooked psychiatrist friend Drake for assistance. The plans of both men are thwarted when Jeffrey's corpse appears and puts a halt to things.