The giant king Bendigeidfran sees ships coming from Ireland. It's Matholwch, the king of Ireland who has an interesting proposal to unite the two families and countries. A feast on Anglesey is arranged but Efnsien, Bendigeidfran's half-brother, is not invited and he takes a bloody and cruel revenge for this insult. This causes problems between the two kings which are solved by Bendigeidfran (the king of the Island of Britain) giving Matholwch (the king of Ireland) a magical cauldron from the Otherworld, the Cauldron of Rebirth.
Matholwch recognises the cauldron and tells the story of how he first came across it and his encounter with two giants and their terrifying children.
Once the cauldron is given there is peace between the two kings and their countries and Branwen sets sail with her new husband, becomes High Queen of Ireland and has a baby boy, who she calls Gwern, after the alder tree.