According to Math fab Mathonwy, the hero Lleu Llaw Gyffes had been placed under a tynged by his mother, Arianrhod, that he may never have a human wife. To counteract this curse, the magicians Math and Gwydion created Blodeuwedd from the flowers of oak, broom and meadowsweet, fashioning her into the most beautiful of maidens.
However, Blodeuwedd is unfaithful and has an affair with the lord of Penllyn, Gronw Pebr. The two lovers conspire to kill Lleu, but Lleu is seemingly invulnerable. As the tale states, he cannot be killed during the day or night, nor indoors or outdoors, neither riding nor walking, not clothed and not naked, nor by any weapon lawfully made. Blodeuwedd manages to trick Lleu into revealing how he can be killed: at dusk, wrapped in a net, with one foot on a bath and one on a goat, by a riverbank and by a spear forged for a year during the hours when everyone is at mass. With this information she arranges his death.
Gronw then ambushes Lleu and strikes him with a spear, but Lleu transforms into an eagle and he flies away. He is later restored to his human self by Gwydion and Math who nurse him back to health and then help him reclaim his lost land from Gronw and Blodeuwedd. Gronw is chased off to his homeland of Penllyn and later killed by Lleu while Blodeuwedd is punished by Math and Gwydion, who transform her into an owl.