The narrative begins with a pillow-talk dispute between King Ailill and Queen Medb of Connacht over their respective wealth, which leads to the discovery that Medb lacks a match for the great Whitehorned Bull. Learning of the magnificent Brown Bull of Cualnge in Ulster, Medb sends messengers to procure it, but negotiations fail when the envoys boast of taking the bull by force. Enraged, Medb musters the four grand provinces of Ireland and launches an invasion. Because the warriors of Ulster are struck down by a periodic divine torpor, the defense of the province falls entirely upon the seventeen-year-old hero Cuchulain.
Cuchulain permits the host to advance only on the condition that they send champions to meet him in single combat day by day. He slaughters a multitude of Connacht's finest warriors, culminating in a devastating four-day battle with his former foster-brother Ferdiad. Meanwhile, the Connacht army breaks chivalric law, overruns the province, and seizes the Brown Bull.
The Ulstermen finally awaken from their debility and gather their forces. In the ensuing battle at Garech, Medb's army is heavily routed and retreats to Connacht. The epic concludes with a titanic battle between the two divine bulls, in which the Brown Bull destroys the Whitehorned before perishing himself.
