In 1684 France, Louis XIV (Pierce Brosnan) - the Sun King - is the most powerful and influential monarch on the planet. But his birthday is approaching along with a rare solar eclipse and he's worried about the future of France. His spiritual advisor, Père La Chaise (William Hurt), comforts him, while his physician, Dr. Labarthe (Pablo Schreiber) informs him that scientists believe mermaids contain a life force which grants immortality. Louis commissions Yves (Benjamin Walker), a young sea captain, to search the North Seas for the heretofore-mythical sea creature. Meanwhile, in order to help pay off war debts, Louis brings to Court Marie-Josèphe (Kaya Scodelario), the daughter of the mistress he always loved. Orphaned at a convent since birth, Marie-Josèphe is educated and musically gifted, yet bears an inherent defiance of authority. Père feels he can use Marie-Josèphe to tap into the deep pockets of Lintillac (Ben Lloyd-Hughes), a wealthy young merchant who is angling for a position at court, as well as use her musical abilities to satisfy Louis' ear. As the week of the Eclipse begins, Yves returns to Versailles, successful in the capture of a mermaid (Fan Bingbing). He is ordered to place her in a reservoir beneath the Grotto of Thetis and restore her to full health. Marie-Josèphe discovers the mermaid, and the two are quick to develop a mysterious ability to communicate. Kindred spirits, they forge an empathic bond based on a common thread: the mermaid, in her capture, has been separated from her children and yearns to be reunited, and Marie-Josèphe still longs to someday know her parents. Marie-Josèphe also falls in love with Yves, not knowing he was the mermaid's captor. Eventually, Marie-Josèphe finds out that Louis has planned not only to marry her off to Lintillac, whom she hates, but worse yet, to kill the mermaid. When she confronts Louis, she learns the secret that has shrouded her entire life: the King is her father. Her heart leaps at the revelation, but the King is more concerned with the future of France then he is in building a relationship with his daughter. In the end, Marie-Josèphe understands she must defy her father, her King, to rescue the mermaid.
