It tells of a vicious war between the north and the south, of the birth of Lleu Llaw Gyffes and Dylan ail Don, of the tyngedau of Arianrhod, and of the creation of Blodeuwedd, a woman made of flowers.

Allusions to the tale can be found in two old triads retained in the Trioedd Ynys Prydain[2] Gilfaethwy, nephew to the Venedotian king, Math fab Mathonwy, falls in love with his uncle's virgin foot-holder, Goewin.

To this end, Gwydion employs his magic powers to steal a number of otherworldly pigs from the Demetian king, Pryderi, who retaliates by marching on Gwynedd.

The two contenders meet at a place called Y Velen Rhyd in Ardudwy, and "because of strength and valour and magic and enchantment", Gwydion triumphs and Pryderi is killed.

When Math hears of the assault on Goewin, he turns his nephews into a series of mated pairs of animals: Gwydion becomes a stag for a year, then a sow and finally a wolf.

During the test, she gives birth to a "sturdy boy with thick yellow hair" whom Math names Dylan and who takes on the nature of the seas until his death at his uncle Gofannon's hands.

Ashamed, Arianrhod runs to the door, but on her way out something small drops from her, which Gwydion wraps up and places in a chest at the foot of his bed.

The boy throws a stone and strikes a wren "between the tendon and the bone of its leg", causing Arianrhod to make the remark "it is with a skillful hand that the fair-haired one has hit it ".

He reveals to her that he can only be killed at dusk, wrapped in a net with one foot on a cauldron and one on a goat and with a spear forged for a year during the hours when everyone is at mass.

Gwydion and Math nurse Lleu back to health before mustering Gwynedd and reclaiming his lands from Gronw and Blodeuwedd.