After five men previously acquitted of various criminal charges are murdered by a mysterious avenger known as Dr. Rx, police Inspector Hurd and Sergeant Sweeney ask private investigator Jerry Church to help them on the case. He takes the case after talking to Dudley Crispin, a brilliant attorney who had defended three of the murdered men. Crispin gains an acquittal for his latest client, Zarini, but the latter falls dead in the courtroom. Jerry marries Kit Logan, who becomes frightened and persuades Jerry to quit the case. He stays however after Ernie Paul, suspected of the Zarini murder, threatens to “get” him if he does not stay on and clear Paul. Dr. Rx captures Jerry and attempts to frighten him into insanity by strapping him on an operating table, and pretending to exchange the detective’s brain for a gorilla.