Kai Ashimoto (足元カイ, Ashimoto Kai) is a solitary and pessimistic ninth grader residing in Hinashi Town (日無町, Hinashi-chō) with his grandfather, who is an umbrella maker, and his father. He is part of a rock band named "SEIRÈN," formed alongside his friends, Kunio and Yūho.
One fateful day, Kai embarks on an adventure to Merfolk Island (人魚島, Ningyojima), where he encounters Lu, a mystical ningyo girl. Through a magical rhythm, Lu's fish fin transforms into human legs, allowing a unique friendship to blossom as Kai introduces her to the town and its wonders. As their bond deepens, Kai discovers that Lu harbors a dangerous secret: she cannot touch sunlight without the risk of bursting into flames. Tragically, Lu's mother was killed by a ship, leaving her father to protect her from the dangers of the world.
During their escapades, Kai introduces Lu to his friends, but in a moment of carelessness, he reveals the merfolk's existence to the townspeople. To his surprise, Lu's enchanting nature and musical talents captivate everyone, making her a viral sensation. Inspired by her newfound popularity, Yūho's grandfather decides to reopen an old merfolk-themed amusement park on a nearby island. However, as Kai starts to lose interest in his band and neglects Lu, a rift develops between them.
On the day of the park's grand opening, tensions escalate when Lu arrives and unexpectedly steals the spotlight, leading to Yūho's frustration and subsequent disappearance. As anti-merfolk sentiments surge in the town, Yūho's father wrongly blames Lu for his daughter's absence and threatens her with exposure to sunlight. Just in time, Lu's father intervenes, but their confrontation inadvertently awakens a curse that threatens to flood the town.
In a heroic effort, Kai and Yūho band together to rescue Lu and her father. The merfolk, alongside Kai and Yūho, work together to save the town from the impending disaster. In doing so, they inadvertently expose themselves to the bright daybreak, leading to their magical reveal as the sun rises. Once the curse begins to dissipate, the townspeople unite with umbrellas in hand, saving the sea creatures from the burning rays of the sun.
Ultimately, Kai seeks forgiveness from Lu, and in a joyous moment, they express their love for one another, as the town celebrates and the sea creatures return to the ocean. As Kai embarks on a new chapter in Hinashi Town, a sense of hope fills the air, affirming that his connection with Lu is far from over and that their paths are bound to cross again in the future.
