Charlie Chan investigates a string of murders having something to do with stolen government radar plans.
A mysterious Chinese woman arrives at Charlie Chan’s home and is murdered there shortly afterward. The only clues are the ring with which she introduced herself, and the message “Capt K” she scrawled before dying.
This was Sidney Toler’s final film and his final one as Charlie Chan. Striken with cancer during his last few films, he was so physically weak during filming that he could hardly walk or say his lines coherently.
Three men are convicted of bank robbery, the main evidence against them being that their fingerprints were found at the scene. However, Charlie Chan believes them to be innocent, and his investigation reveals that they are indeed innocent and that their fingerprints were forged and planted in the prison files to frame them. Charlie sets out to uncover the real bank robbers.