Oddly enough The Black Sleep was some years ahead of its time medically speaking. The title refers to a drug from India that scientists Basil Rathbone uses to do that. Today it's a technique to enable recovery from certain illnesses or injuries. But being that this is Basil Rathbone mad scientist you know the drug will be used for all kinds of nefarious purposes.
Rathbone gets Dr. Herbert Rudley out of prison to assist him by use of his coma inducing Black Sleep. Rudley is in prison for a murder he didn't commit. When he 'dies' before the death sentence is carried out that's the end of it. But Rathbone has a lot of work for Rudley to do, operations on some willing and not so willing patients. What it's all about you have to see The Black Sleep for.
If you do see it you're in for a treat because with a cast of scene stealing actors such as Bela Lugosi, Lon Chaney, Jr., and John Carradine, this is not to be missed. All three of their characters are products of Rathbone's failed experiments. Carradine in particular is joy. He plays a deranged man who thinks he's a Crusader King and he's overacting outrageously and I'm loving every minute of it. Also in the film is Akim Tamiroff as a gypsy grave robber who also aids Rathbone.
All these people have legions of fans still. So if you want to see a film that's a combination of Frankenstein and the Island of Dr. Moreau with a great cast you can't miss with The Black Sleep.