One night in bed, Queen Maeve of Connacht quarrels with her husband Ailill. They argue over who has the most wealth. Maeve taunts Ailill, saying he's a kept man. Ailill's magnificent White Bull is the deciing factor in their measuring of possessions. But Maeve is a bad loser. Mac Roth, her messenger, goes to Cooley to rent the famed Brown Bull for a year, thus giving her the advantage.

The Bull's owner is agreeable until Mac Roth and his party, drunk, reveal that had they not been allowed to borrow the bull they would have taken it by force. The deal breaks down. They go home emptyhanded. Maeve decides on war.

Having marshalled her warriors, allies from Munster and Tara, and with Ailill's six brothers and their armies standing by, Maeve receives favourable omens from her Druids. The long march to Cooley bgins. However, a sorceress appears and warns of impending defeat at the hand of Dearg Doom, Cuchulainn, Ulster's champion.

Maeve ignores the warning. Meanwhile, the men of Ulster are ill with labour pains - the legacy of a curse put on them for their inhuman treatment of a pregnant woman. The one man exempt from the curse is Cuchulainn, whose very birth is shrouded in mystery.

Alone, Cuchulainn takes on the defence of Ulster, harassing Maeve's soldiers, swooping again and again, beheading any who stray from the main force. Cuchulainn, originally called Setanta, became known as Cu Chulainn, the Hound of Culann, because of his savagery.

As the Connacht losses grow, the deposed King of Ulster, Fergus MacRoich, who is having a secret affair with Maeve, meets Cuchulainn and arranges a treaty. Cuchulainn agrees to single-handed combat with any Connacht champion, provided Maeve's army does not advance.

One by one, day after day, Cuchulainn defeats each champion, until eventually he faces Ferdia, his foster-brother and friend. Cuchulainn pleads with Ferdia to leave, but Ferdia refuses, laughing. For three days they fight. On the third, Cuchulainn flies into a heedless killing rage and lets loose his supernatural javelin, the deadly GaeBolga, which destroys his friend.

As Ferdia falls, Cuchulainn catches him and carries him to the riverbank, lamenting. Overcome by despair, Cuchulainn abandons the fight. Maeve's army moves south with the stolen bull. The Ulster men rally and with Cuchulainn in their ranks, give chase.

The Morrigan, Goddess of war, who has been encouraging the slaughter all along, prophesies the outcome. In the battle which follows, the Connacht army is routed. But Maeve's life is spared by Cuchulainn.

As the Ulstermen are taking the Brown Bull home, they meet Ailill's Bull, the White-Horned one. The Brown Bull of Donn immediately attacks the White. All day and night they are locked in terrible combat. Morning sees the Donn victorious. The armies consider destroying him, the cause of all their suffering, but leave him as, dying, he staggers homewards.