kindly dr. john garth ( boris karloff ) be find guilty of a mercy killing and sentence to hang . while in prison he experiment in try to find a serum that halt old age . he use the blood of an executed murderer and try it on himself . he be then pardon and find the serum work . but it turn him into a cold blood killer which he have no control over . this be pretty much a forgotten movie and it easy to see why . it pretty slow and dull with terrible dialogue and questionable science . also it ( technically ) not a horror movie it play more like a melodrama with horrific touch . wonderful actress evelyn keyes be in here too but be give nothing to do . karloff single handedly save this from total disaster . he just great in his role he make the bad dialogue sound like shakespeare . also when the kill urge take over you can actually see the change in his face and mannerism and there be also some nice atmospheric scene at the end . there a funny lapse in continuity keyes find out by the morning paper about a murderer but a scene outside the house show it night time . this be really only for karloff fan . i give it a 4 .