Married college professor Michael Lane falls in love with one of his students and hatches an elaborate scheme to dispose of his wife, Elizabeth. His murder plot goes as planned. However, he soon receives an anonymous note: "I know what you did, murderer!" Prof. Lane frantically tries to discover who knows his secret...

The Bowery Boys tangle with Scotland Yard, diamond smugglers and a gem-toting canine during an ocean cruise.

Confederate super-spy Johnny and his partner in crime Steve travel to San Francisco near the end of the Civil War, masquerading, respectively, as a singer/guitar instructor and a magic-elixir vendor. Once there, Johnny dons a fake wig, beard and mustache, and steals Union gold to bring back to the South, aided by a guitar that doubles as a gun.

A private detective helps a prostitute being assaulted, and notices that she is wearing a very unique ring. She is later found murdered and there is no trace of the ring, which turns out to be part of a cache of jewelry stolen by the Nazis during World War II and smuggled out of France after the wary by an American army colonel. The private eye decides to try to find the ring, is hired by the former colonel to find the rest of the jewels, and runs up against a murderous French gang that is also looking for the jewels.