Gwern's story begins with the marriage of Branwen and Matholwch. The Irish king comes to Britain to seek Branwen's hand, but Efnysien resents not being consulted and mutilates the visitors' horses. Bran attempts to repair the offence with compensation, including the Cauldron of Rebirth. Matholwch returns to Ireland with his bride, and their son is born during the early period of goodwill. Named Gwern son of Matholwch, he is sent to be fostered among distinguished people in Ireland. The tale does not place his birth at Harlech or describe a separate childhood in Britain.

Resentment over the injured horses later returns. Matholwch's household turns against Branwen, who is removed from her position and made to work in the kitchens, where she is struck each day. Restrictions on travel prevent news of her treatment reaching Britain. She eventually trains a starling to carry a message to her brother. Bran brings a great force across the sea, and the threatened Irish offer to transfer the kingdom to Gwern. The proposal uses his descent from both families as the basis for peace. The boy is made central to negotiations whose causes and dangers lie far beyond his own actions.

A hall large enough for Bran is built for the reconciliation feast, but armed men are concealed in bags within it. Efnysien discovers and kills them before the gathering. Despite this treachery, the two sides meet and the kingship is conferred upon Gwern. Bran calls the boy to him, followed by Manawydan and Nisien. The child goes willingly between these relatives. When Efnysien asks why his nephew has not approached him too, the others allow Gwern to go. That movement across the hall gives the episode its force: an act of ordinary family trust takes place in a setting already marked by concealed violence.

Efnysien suddenly throws Gwern into the fire. Branwen attempts to follow her son, but Bran restrains her while the hall erupts into fighting. The cauldron that once helped repair the marriage alliance now restores dead Irish warriors, who return the following day without speech. Gwern is not said to be revived by it. Efnysien eventually destroys the vessel from within and dies in the act, but his sacrifice cannot restore the child or undo the slaughter. The tale does not offer an easy victory to either side, and only seven men return from Bran's expedition.

Bran receives a mortal wound and instructs the survivors to carry his severed head back to Britain. Branwen returns with them but dies near the River Alaw after lamenting the destruction of the two islands. Gwern's brief kingship therefore remains a broken promise rather than a completed reign. His years in fosterage should not be reduced to a life lasting only the span of a feast, and the absence of a burial account does not establish a universal claim that no grave ever belonged to him. Within the surviving story, his loss binds the failed alliance, the feast and his mother's grief into one family tragedy.