During the late seventeenth century, Louis XIV’s natural philosopher and explorer, Father Yves de la Croix, does what no one has done for four hundred years: he brings a living sea monster to land. Thus begins a stunning fantasy, a journey into science and superstition, and an alternate history in which Yves and his sister, Marie-Josèphe—a lady-in-waiting with her own finely tuned intelligence and insatiable curiosity—struggle to learn from and protect the sea woman. As Marie-Josèphe translates the sea woman’s songs into stories, she hopes to stave off the creature’s inevitable execution—for Louis XIV believes the wondrous being holds the secret to the immortality he craves, a twisted obsession that will force brother and sister to choose between their conscience and their loyalty to king and country.