Macha, one of the Keltoi triple goddesses of war during the Iron Age, circa 463 BCE, lays a curse on the kingdom of the Ulaidh in revenge for her humiliation, at their hands, during her birth pangs. Breoga, a wine trader, tells Titus Publius, a junior tribune in Gaul, of the lady Ness, who, with the help of the draoidh Cathbad, tricks Fergus mac Rioch, king of the Ulaidh, into surrendering the kingship in favour of her son, Conor.
Cathbad, the draoidh, prophesies that a triple conceived child, born to Deichtine, the daughter of the usurped king, will be the hero to protect the kingdom in its greatest time of need. Portents at the child’s semi-divine birth herald the arrival of a champion whose name will last forever but whose life will be short and bloody. Signalling his later prowess, the child kills the fierce wolf hound of the blacksmith, Culann and the draoidh announces that the name of Cú Chulainn – the Hound of Culann - would live on forever in the minds of fighting men.
A further prophecy predicts the destruction of Eamhain Macha, the seat of power in the Ulaidh, with the birth of a golden-haired girl, Deirdre of the Sorrows. Conor had sworn then to avert the prophecy by having the child reared far away from the eyes of men and kept for him. What queen, he reasoned, would then see the willing destruction of her own home? Naoise, a warrior sworn to protect the kingdom of the Ulaidh, encounters Deirdre moments before she goes to Conor’s bed and the couple elope, much to the king’s lustful fury.
Their departure leaves the kingdom open to a raid by the neighbouring kingdom of Connachta ruled by the rapacious Medb in her quest for the fabled Donn, the sacred Brown Bull of Cúailnge to match Finnbenach, the massive white horned bull belonging to her husband, Ailil.
Breoga meets up again with Titus Publius, now a tribune, and tells how Cú Chulainn kills his own son, using the cursed gae bolga, a weapon presented to him upon completion of his warrior training in Dal Riata by the Shadowy One.
Cú Chulainn is alerted to the dangers of the invasion when he realises that the curse of Macha - the pangs - have set in, disabling the fighting men of the Ulaidh and he vows to single-handedly protect the territory of the kingdom, setting superhuman tasks in the path of the invading forces in order to slow their advance.
Continually challenged by champions urged on by a wrathful Medb, Cú Chulainn kills all he encounters, spreading fear within the invading forces' ranks.
The war goddess, Macha, visits Cú Chulainn and offers her love, help and support in all his actions but Cú Chulainn rebuffs her advances and scorns her offer before she angrily warns him that she will now hinder him instead. Cú Chulainn continues to block the advance of Medb and Ailil's army by challenging its heroes to single combat at the ford he guards. One by one, the best fall until Medb humiliates and cajoles Ferdia, Cú Chulainn’s foster brother, into finally agreeing to fight his battle equal.
Cú Chulainn, greatly weakened by the fierce combat against his foster brother, the never ending slaughter and the attacks of Macha, tries to rouse the warriors of the Ulaidh, many of whom are still weak, to the defence of the kingdom in the final battle on the plain of Gáirech.
Titus Publius, on the eve of the invasion of Britannia, encounters for the final time, the old wine trader who tells him the end of the final battle and the consequences of the Donn of Cúailnge and the White Horned Bull meeting up.