Middle-aged ship's bosun George is about to marry young and pretty Louella. Her former lover, Skip Clark, is jealous; when George turns up dead, Skip is the chief suspect. Blackie believes Skip is innocent; George was killed for his money.
Phantom Burglar steals Lenore Aldwin's important documents; she offers $500 reward for their return. Crooks want 10 to 20 times that amount of money. Boston Blackie is determined to find out what was so important in those stolen letters.
A small time pickpocket runs afoul of what looks like a sweet old lady in old fashioned clothes with a comically old automobile, but is in fact a ruthless head of a crime ring. He's shot and put into a coma. Blackie senses the lady, a known ex-con, is up to something as she and her mob plan an art museum heist.