Anthony Monday and his family live in Hoosac, Minnesota, and, while not poor, are having money problems. Anthony is desperate to help with expenses and accepts a part-time job from Myra Eells, the elderly librarian of Hoosac Public Library.
Working at the library allows Anthony to earn a little money, as well learn more about the man who built the library, the wealthy and eccentric Alpheus Winterborn. Rumor has it that Winterborn found something on an archeological dig many years before and hid it for safekeeping somewhere in the library, but no one believes the tale to be true.
During his chores around the building, Anthony ultimately finds a clue that hints that the Winterborn treasure does exist and if clues, written by Winterborn himself, are followed correctly they will lead the lucky treasure hunter to the prize. Anthony knows that finding the treasure will result in money that can help with family finances and treatment during his father's heart-attack recovery. But soon Anthony runs afoul of the greedy bank vice-president, Hugo Philpotts, who seems to know a little something about the Winterborn treasure, too.
They both fight for the treasure and look for it. Anthony finds the treasure in the weathervane, a gold statue that was worth a lot of money; after which he sells it. He gave half of the money to Ms. Eells and kept half for himself.