Karloff is a scientist experimenting on bringing the dead back to life (sort of ... he's using a whacky mix of cryogenics and a huge glass artificial heart to either raise the dead or put people in suspended animation). When the police interrupt one of his experiments at the behest of a traitorous nurse, his volunteer dies and he's tried for murder and executed. His assistant brings him back to life so he can seek vengeance on those who did him in. This is an average genre outing elevated by another great Karloff performance.
Review of The Man They Could Not Hang
