The Irish king Matholwch sails to Harlech to speak with Bran the Blessed high king of the Island of the Mighty and to ask for the hand of his sister Branwen in marriage, thus forging an alliance between the two islands. Bendigeidfran agrees to Matholwch's request, but the celebrations are cut short when Efnisien, a half-brother to the children of Llŷr, brutally mutilates Matholwch's horses, angry that his permission was not sought in regards to the marriage. Matholwch is deeply offended until Bran offers him compensation in the form of a magic cauldron that can restore the dead to life. Pleased with the gift, Matholwch and Branwen sail back to Ireland to reign. Once in Matholwch's kingdom, Branwen gives birth to a son, Gwern, but Efnisien's insult continues to rankle among the Irish and, eventually, Branwen is mistreated, banished to the kitchens and beaten every day. She tames a starling and sends it across the Irish Sea with a message to her brother Bendigeidfran, who sails from Wales to Ireland to rescue her with his brother, Manawydan, and a huge host of warriors, mustered from the 154 cantrefs of Britain. The Irish offer to make peace and build a house big enough to entertain Bendigeidfrân but hang a hundred bags inside, supposedly containing flour but actually containing armed warriors. Efnisien, suspecting a trick, reconnoitres the hall and kills the warriors by crushing their heads inside the bags. Later, at the feast, Efnisien, again feeling insulted, throws Gwern on the fire and a savage battle breaks out. Seeing that the Irish are using the cauldron to revive their dead, Efnisien hides among the corpses and destroys the cauldron, sacrificing himself in the process. Only seven men survive the conflict, among them Manawydan, Taliesin and Pryderi fab Pwyll, prince of Dyfed, Branwen having herself died of a broken heart. The survivors are told by the mortally wounded Bran to cut off his head and to return it to Britain. For seven years the seven survivors stay in Harlech, where they are entertained by Bendigeidfran's head, which continues to speak. They later move on to Gwales where they live for eighty years without perceiving the passing of time. Eventually, Heilyn fab Gwyn opens the door of the hall facing Cornwall and the sorrow of what had befallen them returns. As instructed they take the now silent head to Gwynfryn, the "White Hill", where they bury it facing France so as to ward off invasion. Before the end of the tale, a small detour is taken to jab at the Irish. The battle in Ireland has only left five pregnant women, who give birth to five sons. These sons repopulate the island with the only women available on the island: the mothers. With that, the island is restored with human life, and five districts are created because of this incest. Though Ireland today is divided into four provinces, the five mothers and five sons helps to explain the structure of this island as well as poke fun at the Irish people. This story, after the digression, concludes with the retelling of the main points of Branwen's story. The name means "white, blessed raven."
