Branwen is the sister of Bendigeidfran and Dan and Efnisien's half sister. One day, Matholwch comes from Ireland to seek her hand in marriage, and Bendigeidfran gives his consent in the hope it will forge a peace between the two nations. However, he doesn't consult the malevolent Efnisien, who has been a constant thorn in their sides, and Efnisien seeks to satisfy his honour by destroying Matholwch's best horses.

Bendigeidfran atones for Efnisien's reckless deeds, and Matholwch and Branwen leave Wales on good terms. But the people of Ireland seek a harsher punishment for Bendigeidfran, and for three years - without the knowledge of anyone in Wales - Branwen is cruelly mistreated, suffering daily beatings from the cook. She is sad and lonely, but eventually sends a message informing Bendigeidfran of her plight via a starling she has befriended.

Her brother comes to Ireland with his army to rescue her, and a bloody battle ensues, until all but seven of the Welsh soldiers are dead. Among the survivors are Dan and Pryderi.

Branwen returns to Wales with the surviving soldiers, but dies of a broken heart on the banks of the river Alaw.