Karloff is executed in an electric chair for a crime he did not commit. The only witnesses who can prove his innocence are (surprise) scientists who are on the verge of finding a way of bringing people back to life.
Lots of electric hi volt arc and great 30's era lab equipment in the resurrection scene. After he awakens, he has a lovely white offset stripe through his hair. His look as the resurrected man is subtle, haunting, and effective.
Karloff stalks and kills the men who framed him. The film has an interesting balance of light chit chat & wise talking gangsters with very dark and genuinely suspenseful death scenes. Karloff slowly walks towards to men and asks over and over, why did you have me killed, and in their fear they stumble out of windows, in front of trains, and accidentally shoot themselves. Karloff's death stare and sucked in face (he took out his false teeth to hollow out his cheeks - to great effect) scare the bejezus out of the gangsters.
Interesting and unique lighting techniques are used in this film as well, notably when Karloff is giving a piano recital and giving the evil eye to the men in the audience responsible for his death. One by one his eye travels from one to the other and they are illuminated in an eerie glow as the background light fades.
Another key shot is of the female lead - her white medical tunic and head piece appear luminescent and glowing in the silvery black and white film. Her face and the background fill somehow darkens with a subtle change of light while her clothes seem to glow even more. The photographer must have known exactly what effect the slight reduction of light would have in the exposure of the film. It's a very delicate and impressive balance. It's pronounced enough to compare it to the effect used for the costumes in Tron. The contrast of luminous angular shapes compared to the darkened exposed skin captivates me, and I find myself wanting to replicate it into a project of my own.
See the lighting effect here - move ahead to the 2:02 mark.
See the entire film in 8 parts, starting here.