In a lavish 17th-century French court, King Louis XIV is obsessed with his legacy and seeks immortality. Driven by desperation, he commissions his spiritual advisor, Père La Chaise, and trusted scientist Dr. Labarthe to capture a mermaid whose life force is rumored to grant eternal life. The mythical creature is believed to reside in the waters surrounding his palace, Versailles. Meanwhile, the King's spirited and curious daughter, Marie-Josèphe, returns from a convent in blissful innocence of her father's machinations. As she becomes integrated into court life, she soon senses that something deeper and darker looms beneath the surface of the opulent lifestyle that initially captivates her.

As the tale unfolds, Marie-Josèphe accidentally encounters the captive mermaid and is shocked to discover its human-like nature. The two form an unlikely friendship, prompting Marie-Josèphe to question her father's intentions and the moral implications of using the mermaid for the King's gain. Her inquisitiveness leads her into a web of courtly intrigue, power struggles, and hidden truths that challenge her loyalty and shape her destiny. With time running out and the grand solar eclipse approaching—a moment selected for the ritual—Marie-Josèphe must navigate between duty and conscience, choosing whether to protect her father’s throne or save the mystical creature whose fate alters hers irrevocably.