Dr. Cameron (George Zucco) has succeeded in his expierments with a serum which will turn a man into a wolf-like monster and is ready to avenge himself on the men who caused his professional failure. He uses it on his gardener Petro (Glenn Strange) and one after the other is killed by his creation. His daughter, Lenora (Anne Nagel), grows suspicious and confides with newspaper reporter Tom Gregory (Johnny Downs).
A violent criminal called The Butcher is brought back to life by a scientist using electricity. The Butcher becomes an invulnerable mute, and seeks vengeance upon those who caused his execution.
He keeps human life in test tubes, and prowls at night in the skin of an ape!
Dr. Bernard Adrian is a kindly mad scientist who seeks to cure a young woman's polio. He needs spinal fluid from a human to complete the formula for his experimental serum. Meanwhile, a vicious circus ape has broken out of its cage, and is terrorizing towns people.