Young Donal, growing up alone with his mother, Kate, works after school for Good Joe, a local greyhound trainer. As Donal's talent with dogs becomes apparent, he strikes a deal with Good Joe to own a promising greyhound, The Mighty Celt, if he can train it to win three successive races. Meanwhile, each of their lives is profoundly affected by the reappearance of O, an enigmatic figure from the past. Good Joe clearly dislikes O, hating all that he represents, and resents the growing closeness of Donal and O, a friendship that worries Kate. When Good Joe reacts to the growing relationship between O and Donal by reneging on his deal, Donal is forced to learn some harsh lessons about life.