Karloff is Dr. John Garth who is on trial for a mercy killing. He is sentenced to be hanged in thirty days, but allowed to keep experimenting in prison under close guard.
His experiments concern stopping the aging process, and he is close to success. He experiments on himself after mixing the blood of a killer with his serum. Just after the injection his sentence is commuted to life in prison. His work goes on, but he soon finds that he has become a "Jekyll and Hyde".
The blood of the killer takes over and he is compelled to strangle people. This is rather reminiscent of the later Karloff thriller "The Haunted Strangler" which would come out 18 years later.
This is good stuff for horror fans and Karloff fans in general. Mill Creek brings us a top notch print from the Columbia library. I hope Mill Creek releases more of this kind of classic material. Recommended!