They get married, but peace eludes them, thanks to Efnisien's wickedness. Branwen is beaten by the cook daily, and treated more like a maid than the queen she is, in order to avenge the shame that Ireland had borne. Bendigeidfran hears about the way she is being treated after Branwen befriends a starling and uses it to send a message to her brother. He then crosses the sea with his mighty army.

A bloody battle follows, in which all but seven of the Welsh soldiers are killed - Dan and Pryderi are amongst the survivors. Witnessing the destruction wreaked on both countries causes Branwen to die of a broken heart on the banks of the river Alaw.