Wall Street broker Robert Cain, Sr., is jailed for embezzling. His college graduate son Bob then turns to crime to raise money for his father's release. As assistant to mobster Mickey Dwyer, then falls for Dwyer's girl Lucky. He winds up in the same prison as his father.

In order to win back his girlfriend, Mike Shayne promises to give up his detective practice and get a job as riveter in an aircraft plant. He quickly finds himself investigating the theft of industrial diamonds from the plant's safe and, utilizing a variety of false identities, traces them first to a dress factory and later to a Hawaii-bound ocean liner. Escaping several attempts on his life, he is able to uncover a Nazi smuggling ring, but the location of the missing diamonds continues to elude him.