George Zucco plays University professor Dr. Alfred Morris, who has discovered an ancient Mayan gas that was used to control the human sacrifices made in their rituals. David Bruce plays his student Ted Allison, who is recruited to be his assistant for the summer. Unfortunately for Ted, his professor is quite ruthless, and uses him as an experiment to try his recreation of the Mayan gas on. He succeeds, but Ted turns into the "mad ghoul" who helps the doctor rifle graves to cut out their hearts for more experiments in overcoming the gas, though this won't do poor Ted much good... Evelyn Ankers has a small role as his girlfriend. Good "ghoulish" premise, but film is strangely meandering and unconvincing, not to mention overly talky and dull. Good cast makes it semi-watchable, but film still fails, though does have a poetic final fade-out.