Sinder weaves together the parallel stories of Ceit, grown to adulthood and having taken her rightful place as Matrarc of the Society, and her great-grandmother, Ainsley Robertson in 1924 Ireland, and the inception of the community that would come to inhabit Sinder Avenue. In 1996, Venice Beach, Ceit struggles with the confines of superstition and tradition that have defined the Society since its start. As her younger brother, Alan, turns eighteen, Ceit enacts a plan to end the Society for good. And in doing so, releasing Alan from what she sees increasingly as a toxic and antiquated way of life as well as freeing herself from her mortal ties to the human realm. In 1924, Ainsley Robertson lives in a secluded community outside of Cork, led by her father, an abusive and cruel ceannaire. Ainsley's story opens with her expecting a baby in a world where being an unwed mother is a criminal act. Ainsley grows to realize her own gifts and connection to the world of the fae as she navigates a world dominated by dogma and oppression. As the sparks of what will become the Society grow to a flame, Ainsley becomes the Matrarc of her people, leading them to a new world, and a new tradition.