Aillil and Medb assemble their army in Cruachan. As they prepare they discuss the main differences between them and realize that the only difference of any significance is that Aillil owns a highly prized bull, Finnbhenach, who had once belonged to the Queen but did not like being in a herd owned by a woman and so took it upon himself to transfer into the King's herd. Medb therefore decided to claim the equally potent bull Donn Cuailnge from his owner to make her wealth equal to that of her husband. She makes a successful agreement to rent the bull for a year and all is going well until her messengers get drunk and accidentally let slip that if the agreement had not been reached they would have taken the bull by force anyway. The deal quickly breaks down and Medb raises an army that included Ulster exiles and other allies, and they set out to capture the bull.
The men of Ulster are debilitated by nine day flu which legend has it is the curse of the goddess Macha who imposed it as revenge upon the King of Ulster who forced her to participate in a chariot race whilst she was heavily pregnant. There is only one person in all of Ulster healthy enough to defend the border - a seventeen year old young man named Cu Chulainn. Because he is not watching the border as he is meant to the invaders actually make it into Ulster but he then waged a guerrilla campaign against them with the help of his charioteer, Laeg. He halts the army by invoking the right of single combat at fords, defeating champion after champion in hand to hand combat that lasts for months.He is aided by various supernatural figures but sometimes they turn out to be more of a hindrance than a help. The Morrigan takes in the form of a beautiful maiden and offers him her love but after he rejected her she takes on the form of an eel, and trips him in the ford and then becomes a wolf who stampeded cattle across the ford. He fights her and wounds her and when she appears to him as a milk maid with the same wounds that Cu Chulainn had caused, he heals her with the milk she has just obtained from the cow. He cannot, however, keep Medb from re-capturing the bull.
Occasionally Medb breaks their single combat agreement and sends several men to fight him at once. Fergus, his foster father, is sent to fight him and Cu Chulainn agrees to yield to him if Fergus agrees to yield to his foster son the next time they meet. There is also an emotionally grueling and lengthy fight between Cu Chulainn and his foster brother Ferdiad. Cu Chuliann emerged victorious and kills Ferdiad with the Gae Bolga, a legendary spear similar in importance to King Arthur's Excalibur.
The debilitated men of Ulster start to recover and the final battle begins. To recover from his wounds Cu Chuliann does not participate at first but has to enter the fray when Fergus has Cochobar at his mercy and when he does, Fergus is forced to make good on their agreement that he should yield to his foster son the next time they meet. Medb is forced to retreat although she does manage to take the bull Don Cuailnge back with her where he fights Finnbhennach and kills him, but in the process becomes mortally wounded himself. He walks around Ireland creating place names before finally returning home to die of exhaustion.
