The deep-sea mermaid clan relies on the "Heart of the Deep" to maintain the barrier protecting their waters. As the once-a-century Ocean Sacrifice approaches, a princess must be chosen to marry into a powerful human conglomerate to ward off a curse. Ariel, the rightful mermaid princess, falls victim to a scheme orchestrated by her cold-blooded father and ambitious younger sister, Selena; not only does Selena steal the credit for saving the Ocean Prince Kane’s life, but Ariel is also slated to be the sacrificial pawn married into Kane’s family to appease the curse. Seeing through the plot, Ariel preemptively switches the marriage arrangement, choosing instead to wed Ryder—a reclusive oceanographer publicly known to be disabled. After the wedding, she uses her mermaid powers to gradually heal Ryder’s legs, and the two fall deeply in love. Meanwhile, Selena’s deception begins to unravel; Kane eventually discovers the truth about his true savior and, consumed by intense remorse, becomes obsessively determined to win Ariel back. Ultimately, Selena’s lies are exposed in public; disgraced, she is stripped of her mermaid identity. When the crisis of the Ocean Sacrifice strikes, Ariel awakens her royal lineage to avert the disaster and ascends to the throne, living out her life with Ryder—who has regained his health and is revealed to be a powerful, hidden figure of the deep. Kane, however, is left to spend his remaining days in solitude and endless regret.
