When Matholwch, king of Ireland, comes to Wales to ask for Branwen's hand in marriage, the giant Bendigeidfran gives his consent, in order to forge a lasting peace between the two nations. But Efnisien isn't consulted, and he is furious to be shamed in such a way. He seeks revenge on the Irish by cutting off the lips, ears, tails and eyelashes from their horses, rendering them worthless.

To atone for his half brother's deeds, Bendigeidfran offers Matholwch his most treasured possession, a magical cauldron that brings dead soldiers back to life. Matholwch leaves Wales happy that his honour has been satisfied, but the Irish still feel that Bendigeidfran hasn't done enough.

For three years, Branwen is punished, until Bendigeidfran hears about the way she is being treated. A bloody war ensues, in which Efnisien dies after sacrificing himself by stretching his body to break the cauldron into little pieces, thus breaking his heart.