The series follows two women—medieval Alaïs Pelletier du Mas (Jessica Brown Findlay), who lives through the Crusades and Cathar massacres in medieval France, and modern-day Alice Tanner (Vanessa Kirby)—in their quest to find the Holy Grail. Alice, a volunteer at a French archaeological excavation, discovers the skeletal remains of two people in a cave, as well as a labyrinth-engraved ring, which attracts the attention of unscrupulous individuals. In 1209, newly married Alaïs is living in Carcassonne, a stronghold of Cathars who have been declared heretical by the Church. Alaïs and her father are protecting three sacred books that reveal the secret of the Holy Grail from the Crusaders.