During the Hundred Years War, Gaston Phébus, Count of Foix and Lord of Béarn, married Myriam, his tender childhood friend with whom he was madly in love. He meets the king of Navarre, Charlemagne, and his sister Agnès, who falls madly in love with the handsome Gaston. With the help of her brother, she poisons Myriam and tries to seduce Gaston. Phébus takes revenge on Agnès by marrying her, but their love life is marked by tragedy and deceit.