Charlie Chan, with son Jimmy on a week's pass from the Army, takes up a request for help at a castle-home, miles from anywhere in the American desert south-west and inhabited by an eccentric, reclusive historian and his wife, a descendant of Lucrezia Borgia. Once there, he finds the request's legitimacy denied by all who are present, but still necessary as one houseguest has already been murdered, the other guests are at each other's throat, and the Borgia-related chatelain is suspected...

To share expenses unemployed Alabama move in with also unemployed Bill and Toodles. Bill is hired by a gangster's mistress and ultimately becomes the gangster's bodyguard. Alabama unknowingly applies for a stenographer's job at Mr. Weber's (the gangster's) business. Bill is forced to fly a plane carrying narcotics into the U.S. but fights back.