Karloff is Dr. Savaard, a doctor who is conducting strange experiments on bringing the dead back to life. Savaard is misunderstood by the authorities and when a test subject dies he is sentenced to die by hanging. Savaard vows revenge on those that have done him wrong, and that is where the crux of the tale lies.
His assistant manages to bring Savaard back to life and he traps several of the jury members and the judge in his home, and plans to kill them one by one. Savaards' daughter Janet has no idea what her formerly dead father is doing until she goes to the old house and discovers everyone is trapped there.
This adds a complication to Savaard's plans. Janet is played by the gorgeous Lorna Gray and it is always a pleasure to see her in any film.
If you get a chance you might want to check this out. It's a great Karloff performance and a great tale mixing murder and sci-fi together.