In
Ystrad Tywi, an unnamed girl lives with her mother Elen. Elen tells the girl stories about the
Tuath Dé and
their treasures. Elen implies that the girl is the daughter of one of the Tuath, and that Elen stole Dagda's Cauldron before fleeing into the wilderness with her daughter. As the girl grows, she develops the ability to move unseen and to communicate with animals. She uses these abilities to save a group of knights from bandits; these knights are Companions in service to
King Arturus of
Caer Leon. The girl decides to leave her mother and explore the world, searching for something she isn't sure of. Elen names her Peretur after the Tuatha's spear; she then casts a
geas which prevents Peretur from speaking of her childhood. Peretur disguises herself as a boy and travels toward Caer Leon, following visions of a magical lake. She challenges
Cei, the king's reeve, and disarms him; he presents her before Arturus and
Gwenhwyfar. Arturus declines to accept Peretur as a knight because she is unable to reveal her parentage. Peretur meets
Nimuë, Arturus's chief counsellor and the keeper of the lake that Peretur has been seeing in her visions. Entering Nimuë's magical realm, Peretur accidentally breaks Elen's geas. Nimüe reveals that
Myrddyn, the king's sorcerer, was seeking the treasures of the Tuath Dé. While powerful, these treasures will eventually corrupt their mortal owners. Myrddyn helped Arturus to find the Sword
Caledfwylch as well as the
Stone, which can seal things away from the mortal world. Nimüe was horrified by Myrddyn's lust for power, so she trapped him in a magical coma and sealed him within the Stone. Nimüe plans to remove the treasures from the world to protect humanity. Nimüe and Peretur make love. Peretur learns that her father
Manandán is seeking both Elen and the Cauldron. Elen sends out a cry which is sensed by many mortals; Arturus's Companions begin having dreams of the
Holy Grail. Peretur tells Arturus that she is Myrddyn's nephew; he allows her to accompany Nimüe and
Llanza to search for the Grail. The group rides to Peretur's childhood home and find that Elen has committed suicide rather than be found by Manandán. Peretur and her father fight; she kills him with his own spear and claims the weapon for herself. Nimüe is stabbed by Manandán in the commotion, but Peretur heals her with the power of the bowl. With his sister Elen's death, Myrddyn dies as well. The searchers return to Caer Leon and tell Gwenhwyfar to drink from the bowl in order to heal her infertility. In truth, Nimuë and Peretur have replaced the bowl with a fake. The real one is sealed with the Stone in order to keep Arturus from producing an heir. Nimüe believes that Arturus would have spawned a line of tyrannical rulers, but allowing his lineage to die out will bring peace. Peretur holds Nimüe in her arms as they discuss whether they made the right choice. ==Major themes==