One of the Great Tales from the ancient Celtic tradition. This story tells of how Fionn Mac Cumhaill, the great Fianna leader, who wishes to marry Grainne, but she will not have him. Instead she runs off with Diarmaid. Then begins the great chase and the final battle, involving mixed loyalties, a warrior's honour and a woman's tragedy.

This story is part of the Celtic inspiration for the romance Tristan and Isolde, which is a cornerstone of the European literary tradition. Here the story is told very simply, remaining true to its original Gaelic version.