An incredible robbery of $50,000 from an oil company spurs the JSA to the conclusion that the Invisible Band is responsible. However, the charwoman who cleans the apartment the JSA meets in, discovers a sketch (by Dr. Mid-Nite) that looks like a puzzle, so she gives it to her nephew, Elmer, who fancies himself an amateur detective. He reaches the same conclusion as the JSA, so disguises himself to investigate the apartment, believing it to be the HQ of the criminals. When the JSA returns, he believes them to be the crooks dressing as the JSA and leaves to report this to his detective school.
Later the JSA ends up being held prisoner by the Invisible Band, who gives Elmer his diploma IF he can put a bullet into seven targets on a stage. Elmer doesn’t know that a JSA member is behind each target, but as he prepares to shoot, he stumbles, wild shots hit a water pipe and the JSA gets free and rounds up the gang.