Queen Medb and King Ailill of Ireland are having a bit of a marital spat. Ailill is congratulating himself for having drastically improved his wife's standard of living. "Says you," retorts Medb, maintaining that she brought just as much wealth to the marriage as he did. So they decide to count it all up. And lo! It comes out exactly equal. Exactly? Not quite. Ailill has everything his wife has plus an exceptional bull. Crafty Medb hatches a plan. As it happens, there is another bull in Ireland that's as quality as Ailill's. It belongs to Dáire mac Fiachna, so she sends a messenger to him asking to borrow it, promising to return it PLUS any calves it gets PLUS she'll have sex with him (Dáire mac Fiachna, not the bull). Who could resist? But, unfortunately, just as Medb's messengers are getting ready to return with the good news, one of Dáire's servants overhears them say that they'd have taken the bull with or without its owner's permission. Of course you realize this means war. Dáire backs out of the deal, and Medb gathers her forces to invade Ulster and take the bull. Unfortunately for Dáire, all of the Ulstermen are suffering from a mysterious illness caused by an ancient curse and can't defend the land. Their only hope is seventeen-year-old hero Cú Chulainn, who is not affected for reasons that are never satisfactorily explained (but have been rationalised in various ways, his youth - not being a man yet - and having been born outside Ulster being popular). Cú Chulainn doesn't start out very well: he is on border duty when the invaders come, but is too busy chasing tail and lets them cross unopposed. When he finds out what's happened, he sends his dad to warn the king, Conchobar, and starts to harass the army with his sling, before halting their advance by demanding single combat at river crossings. A series of these combats follow - Conchobar doesn't show up with his army for months, thanks to the curse - and Cú Chulainn wins them all, although with increasing difficulty. Eventually, Fergus (his foster-father and former king of Ulster in exile) is sent to fight him, but neither he nor Cú Chulainn can bring themselves to fight each other, and in any case Fergus has no sword - Ailill stole it while he was in flagrante with Medb, and Fergus carved himself a wooden dummy one to conceal the fact. They agree that Cú Chulainn will yield this time, and Fergus will yield the next time they meet, and the army advances to the next river, where the combats resume. After one such combat, Cú Chulainn lies wounded, and is tended by the god Lug, who turns out to be Cú Chulaimn's biological father. While he's out of action, the boys of Ulster awake from the curse, attack the invaders, and are slaughtered. Cú Chulainn, now fully healed, returns to the fray, and seeing what's happened, goes into a ríastrad or "warp-spasm", becoming a hideous, unrecognisable monster who doesn't know friend from foe, and revisits the slaughter on the Connacht army sixfold. Then, as if nothing has happened, the single combats resume, and a new champion is sent to fight him - Fer Diad, his best and most intimate friend from their days training in arms under the warrior woman Scathach, who has mysteriously never been mentioned before. Medb had to get Fer Diad stinking drunk and goad him into saying some things about Cú Chulainn he couldn't take back before he'd agree to fight him. They fight for three days, the early chivalry, sportsmanship and fond reminiscences of the times they used to share a bed eventually giving way to grim, silent struggle. After Fer Diad has been on top for a while, Cú Chulainn loses control, has another warp-spasm, and finishes it by ramming his special spear, the one Scathach taught only him to use, right up Fer Diad's arse. After lamenting his dead chum, Cú Chulainn sits things out for a while, recuperating from his new wounds. Ulster warriors come to his aid in ones and twos, until his dad (although not, as we now know, his real one) finally makes it to king Conchobar and warns him of the invasion, but because he didn't follow the correct protocol he's about to be put to death. He tries to run away but only manages to decapitate himself on the sharpened edge of his own shield. But his severed head continues to cry out his warning to Conchobar, who finally gets the message and raises his army. Big battle. Fergus gets Ailill to give him his sword back and advances against Conchobar, who he hates for murdering someone he'd sworn to protect, and is just about to kill him when another of his foster-sons, Conchobar's son Cormac, stops him. Instead, he cuts off the tops of two nearby hills with one stroke of his sword. Cú Chulainn, seeing Fergus advance, re-enters the fray despite his wounds and demands Fergus live up to their bargain and yield to him. Fergus agrees and pulls his forces off the field. The rest of the Connacht army see this, panic, and start to retreat. In the middle of all this, Medb has her period (although the monk who wrote it, not being overly familiar with female biology, gets a bit confused between blood and urine), and Cú Chulainn comes upon her as she's trying to clean herself up. He decides that, as he doesn't usually kill women, he'll have to protect her, and guards her retreat back to Connacht—with the bull, which had been captured at some point previously. So Medb has lost the battle but gained her bull. But when she gets it home it takes one look at Ailill's bull, and it takes one look back, and they fight. The bull of Cooley kills Ailill's bull and wanders Ireland for a bit, creating placenames with bits of the dismembered carcass, before dying of exhaustion. So now nobody has a bull—Medb and Ailill are finally equal. That's what happens, says Fergus, when a herd of horses is led by a mare, conveniently ignoring his own divided loyalties which were a major cause of the shambles the expedition turned into.