Since infancy, Marie-Joseph, daughter of the "sun King" Louis XIV, was brought up in a convent, but the strict nuns never managed to instil in her either obedience or humility. When the already middle-aged king decides that he has no one to leave France to, and therefore he must live forever, a court scholar finds a solution - a mermaid caught in the sea is brought to the palace, who has the source of immortality. In the meantime, Marie-Joseph, who is unaware of her kinship with the monarch, is summoned to court and an incredible bond is soon established between her and the captive mermaid.