Dr John Garth (Boris Karloff) has been sentenced to death for his part in a mercy killing, where he ended the life of a terminally ill patient. Dr Garth fears that now his life's work, a search for a serum that will reverse the ageing process, will never be completed, but the prison warden allows him to continue his experiments for what little time he has left. Using blood from a recently deceased murderer, Dr Garth tests the serum on himself, but when his sentence is commuted to life imprisonment at the last minute, he finds that the serum has indeed made him younger... and filled with murderous intent...
This melancholy shocker, written by Robert D. Andrews, was one of a handful of doomed scientist horrors Karloff and director Nick Grinde made together, but is also one of many misguided scientist chillers that Karloff settled into after Frankenstein. The role is as familiar as the storyline: scientist sets out to save lives or cheat death, but ends up surrounded by corpses and on the road to Hell by the finale; not that this predictability harms the films, in fact, it's part of their appeal.
Dr Garth sees death as a disease for which there is a cure. He wishes his serum to be used on great men who are getting on in years so that the world can benefit from their presence for longer; the younger generation are represented by his bright-eyed daughter (Evelyn Keyes) and her bland boyfriend (Bruce Bennett), neither of whom are marked for greatness (despite the apparent optimism of the epilogue). Dr Garth's problem is that he can't bear to imagine the world without himself, his talented friends and their peers; he learns the hard way that you have to let go eventually.
Developing the serum from murderer's blood has echoes of the "bad brain" from Frankenstein, and its effects are much as you'd expect. After bumping off the prison doctor (Edward Van Sloan) and blaming it on a slow witted janitor who he also kills, apparently in self defence, Dr Garth sets out on a reluctant killing spree, only slowly realising what he has become (we know he's going to kill someone because he rubs the back of his neck and takes out his large handkerchief for strangling purposes). Essentially, he has become what he was convicted of - he can't escape his fate, and the film's sorrowful tone makes it clear that no one can.