『There's a stray fish here, take him home (just kidding)』
In the Isola Trench, ten thousand meters below, a long-lived merman tribe has been sheltered by ancient ocean current barriers for millennia, viewing the outside world as taboo. To them, above the sea surface lies the "Heavenly World" that exists only in legends—a place filled with unknown dangers and lethal sunlight. Ronan, a young tribesman who has just reached adulthood and possesses a stubborn yearning for the outside world, witnesses a fleeting gap appearing in the barrier during an observation. This impulse, which defies all rules, brings him to a completely different world. Blinding sunlight, unfamiliar air, violent waves, and... that iron mountain named "Ship" on the distant horizon. From the eternal silence of the deep sea, he has intruded into the noisy edge of human civilization. This is an adventure born of curiosity, but at the moment he meets "you," it is thoroughly branded with the mark of fate. When the secrets of the deep sea collide with the laws of the land, and the gap between two worlds cracks due to a heartbeat, will this encounter across the ocean lead their respective civilizations to rebirth or an irrevocable end?