The story proper begins with Ailill and Medb king and queen of Connacht, who compare their respective wealths and find that the only thing that distinguishes them is Ailill’s possession of the phenomenally fertile white bull called Finnbhennach. He had been born into Medb’s herd of cattle but scorned being owned by a woman and so decided to transfer himself to Ailill’s herd. Medb determines to get the equally potent Donn Cuailnge, a brown bull from Cooley in the province of Ulster, to balance the books and to have equal possessions with her husband. She successfully negotiates with the bull’s owner to rent the animal for a year until her messengers, drunk, reveal that they would have taken the bull by force even if they had not been allowed to borrow it. The deal breaks down, and Medb raises an army of thousands and sets out to capture him.
The men of Ulster are determined to fight Medb’s army but are disabled by a curse and fall into a deep sleep. The only person fit to defend Ulster is seventeen-year-old Cú Chulainn but he lets the army take Ulster by surprise because he’s off on a tryst when he should be watching the border. Medb takes the bull, but Cú Chulainn prevents her from taking him back to Connacht by invoking the right of single combat at fords. He defeats champion after champion in a stand-off lasting months. When Fergus, his foster-father, is set to face him, Cú Chulainn agrees to yield to him on the condition that Fergus yields the next time they meet. Finally there is a physically and emotionally gruelling three-day duel between the hero and his foster-brother and best friend Ferdia, and this ends up with Cú Chulainn killing his best friend and step-brother
Eventually the sleeping Ulstermen start to rouse, one by one at first, then en masse, and the final battle begins. It ends after Fergus makes good on his promise and yields to Cú Chulainn, pulling his forces off the field. Connacht’s other allies panic and Medb is forced to retreat. She does, however, manage to bring Donn Cuailnge, The Brown Bull, back to Connacht, where he fights the white bull, Finnbhennach at Athlone. He kills him, but, mortally wounds himself. He wanders around Ireland where many place names are named after him, before wandering back to Cooley where he dies of exhaustion.
The Tain is one of the great epics of Irish Literature.
The moral of the epic is the futility of war and the worthlessness of possessions.