Niamh travels to the late summer festival known as the Lughnasa Fair, a great gathering of the kingdoms. There she meets Bryan, a member of the Fianna, a group of the king’s finest fighting men. He wishes her to be his “Lughnasa Sister”—his mate for the fourteen days of the Fair—but Niamh wants an offer of marriage. And the offer must come from a man who can answer her riddle: What three things does a woman want most from a man?

Yet Bryan has little time to think on Niamh’s riddle, for the Fair is plagued by a supernatural creature: the puca—a malevolent, destructive spirit in the shape of a black horse with fiery red eyes. Putting aside other matters, Bryan and Niamh must work to solve the mystery of the puca and save the people of the Fair—and their future—together