On the trail of some industrial diamonds that go missing from the safe of his new employers, loved-up but down on his luck detective "Mike Shayne" (Lloyd Nolan) begs and borrows some cash so he can travel on an ocean liner upon which he is sure are the thieves. Pretty soon the presence of "Helen" (Helene Reynolds) arouses his suspicions - she isn't a lady to me messed with, but he also has a new ally in "Juan" (George Reeves) so has hopes of getting to the bottom of what looks increasingly like a Nazi plot. The boat scenario provides plenty for the story: small portholes, foggy decks and…