Screen legend Boris Karloff stars as former opera star Gravelle, now confined to an insane asylum. Seeking revenge on his wife, Gravelle escapes with plans of a murderous rampage at the opera house. Detective Charlie Chan (Warner Oland) and Sgt. Kelly (William Demarest) are at odds with each other about Gravelle's guilt and alleged insanity. Charlotte Henry appears as Gravelle's daughter, and Keye Luke portrays Chan's son, Lee.