Blake Drywater and his good friend Donald Poindexter have another strange teacher as a substitute in one of their classes. Tiny little Mr. Flarf looks and acts just like a typical leprechaun, even down to his flowing beard and pot of gold. Mr. Flarf claims he can take them back in time to see one of Blake's mysterious ancestors, who has a shady past.
But when Mr. Flarf takes the boys to the underground time tunnels of the leprechauns, things are once more not as they expected. While the tunnels are full of doors that can take them to any time or place, they also find a strange hourglass and frightening doors that open into unknown future possibilities. And then Blake discovers the truth about the leprechauns and their time tunnels, and why he was brought there—a discovery that could change the world!