Princess Guinevere has come to Camelot to wed a stranger: the charismatic King Arthur. With magic clawing at the kingdom's borders, the wizard Merlin conjured a solution—send in Guinevere to be Arthur's wife and his protector from those who want to see the young king's idyllic city fail. Guinevere's real name and her true identity are a secret. She is a changeling, a girl who has given up everything to protect Camelot.

To keep Arthur safe, Guinevere must navigate a court in which the old—including Arthur's own family—demand things continue as they have been, and the new—those drawn by the dream of Camelot—fight for a better way to live. Always, in the green hearts of forests and the black depths of lakes, magic lies in wait to reclaim the land. Deadly jousts, duplicitous knights, and forbidden romances are nothing compared to the greatest threat of all: the girl with the long black hair, riding on horseback through the dark woods toward Arthur. When an entire existence is a lie, it is difficult to know who to trust.