In the enchanted depths of the Moonstone Sea, seven merman princes rule the ocean realms. Among them, Prince Yoongi, the quiet sorcerer, wields ancient sea magic in solitude—until one stormy night, he rises to the surface and discovers a human baby drifting from a wrecked ship.

Drawn by an unexplainable bond, Yoongi defies ocean law and transforms the child into a half-merman, naming him Sunoo. Raised in the coral palace, Sunoo grows radiant—crimson-haired, red-tailed, joyful and mesmerizing. Yoongi, once cold and distant, grows fiercely protective, his love for Sunoo blurring the lines between devotion and something deeper.

"I gave you life beneath the waves," Yoongi swears, "and I’ll protect you from any darkness, even if it consumes me. No one will take you from me—not while I breathe."

Far across the sea sails the legendary trade ship En-Sail, carrying six human princes—Heeseung, Jungwon, Jay, Jake, Sunghoon, and Niki. One golden dusk, Heeseung, the eldest, glimpses Sunoo leaping through the waves—wild, beautiful, otherworldly.

From that moment, Heeseung is consumed.

"He’s not just a vision," Heeseung murmurs. "He’s the only fire that can melt my frozen soul. I’ll cross any storm to claim him."

As Heeseung’s obsession deepens and Yoongi’s grip tightens, Sunoo is caught between two tides—a world that raised him in love, and a prince who would burn kingdoms for him. Prophecy stirs, hearts clash, and in the war between sea and shore, Sunoo's choice could either unite or destroy them all.