Young Donal (Tyrone Mckenna), growing up alone with his mother, Kate (Gillian Anderson) in Belfast, works after school for Good Joe (Ken Stott), 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.

When O (Robert Carlyle), a strange and brooding figure from the town's past, reappears unexpectedly, his friendship with Donal begins to trouble Kate. She lets the friendship grow but when Good Joe goes back on his deal and tensions explode between him and O, Donal is forced to learn some harsh lessons about life and risks losing The Mighty Celt altogether.