Young Barnaby Grimes is a "tick-tock" lad making deliveries of all natures to all parts of the city. He prefers to travel about by "high-stacking," which means he travels the rooftops instead of the roadways. His mission brings him to the door of one Dr. Cadwallader who hires Barnaby to deliver letters to some of the most alone and needy folk. Barnaby's good sense causes him to suspect the doctor of foul deeds. By the light of the full moon, he discovers the truth.