Peretur meets Nimuë, Arturus's chief counsellor and the keeper of the lake that Peretur has been seeing in her visions. Arturus declines to accept Peretur as a knight because she is unable to reveal her parentage. She challenges Cei, the king's reeve, and disarms him he presents her before Arturus and Gwenhwyfar. Peretur disguises herself as a boy and travels toward Caer Leon, following visions of a magical lake. Elen names her Peretur after the Tuatha's spear she then casts a geas which prevents Peretur from speaking of her childhood. The girl decides to leave her mother and explore the world, searching for something she isn't sure of. She uses these abilities to save a group of knights from bandits these knights are Companions in service to King Arturus of Caer Leon. As the girl grows, she develops the ability to move unseen and to communicate with animals. 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. Elen tells the girl stories about the Tuath Dé and their treasures. In Ystrad Tywi, an unnamed girl lives with her mother Elen. It also draws heavily on Celtic and European mythos. According to a review in Strange Horizons, the Welsh orthography of several characters' names, including Gwenhwyfar and Myrddyn, conjure a 'Celtic Arthur', providing a geographic explanation for the presence of the Irish Tuath Dé. The book draws on Arthurian mythos, but makes the character of Percival a woman. It uses a similar setting, being placed in a region corresponding to Northumbria, during a period in which the English were being converted to Christianity.

Griffith wrote Spear while taking time off of writing a sequel to her 2013 novel Hild.

The idea for the book originally came about after Griffith was invited to contribute to Sword Stone Table, an anthology of Arthurian retellings from Swapna Krishna and Jenn Northington. It is a queer feminist retelling of the myth of Percival and the Holy Grail. Spear is a 2022 fantasy novella by Nicola Griffith.

Spear by nicola griffith