 After five men previously acquitted of various criminal charges are murdered by a mysterious avenger known as Dr. Rx, police Inspector Hurd (Edmund MacDonald) and Sergeant Sweeney (Shemp Howard) ask private investigator Jerry Church (Patric Knowles) to help them on the case. He takes the case after talking to Dudley Crispin (Samuel S. Hinds), a brilliant attorney who had defended three of the murdered men. Crispin gains an acquittal for his latest client, Zarini (Matty Fain), but the latter falls dead in the courtroom. Jerry marries Kit Logan (Anne Gwynne), who becomes frightened and persuades Jerry to quit the case. He stays however after Ernie Paul (John Gallaudet), 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 (Ray Corrigan in his gorilla suit).