In a mystical realm hidden beneath the human world, where magical beings thrive, a young girl named Chun embarks on a pivotal journey during a coming-of-age ritual. She is transformed into a red dolphin and must navigate the human world for a week. It is here that she meets a human boy who cherishes the ocean's creatures. Tragedy strikes during a storm when Chun gets caught in a fishing net, leading to the boy's death as he heroically attempts to save her. Heartbroken, Chun returns to her realm clutching the boy's ocarina.
In her desire to revive her friend, Chun pleads with the Soul Keeper, a being responsible for guiding virtuous souls from the human world. In a desperate bargain, she willingly sacrifices half her lifespan in exchange for the boy’s essence, which is reborn as a baby dolphin named Kun. With the help of her childhood friend Qiu, Chun faces the challenges of raising Kun despite the taboo against human beings in her world.
Chaos ensues when Chun's mother discovers Kun and cruelly disposes of him into the sewer. Determined to rescue him, Chun and Qiu encounter a rat matron who is intrigued by Kun and aims to reclaim him with her rat minions. Despite their best efforts to protect Kun from the villagers’ wrath, Qiu suffers a perilous bite from a two-headed snake, prompting Chun's grandfather to sacrifice himself to save the boy. His spirit endures through a magnificent begonia tree, which later plays a crucial role.
As strange, unnatural storms besiege Chun's realm—signs of Kun's disruption—the local elders and Chun’s family swiftly recognize the cause. One night, warned by the rat matron, Qiu decides to hide Kun in a frozen lake to shield Chun from danger, but her instinct drives her to plunge into the icy depths after him. The rat matron summons her rats to rescue both from the chill, but she seizes the ocarina with plans to enter the human world herself.
Amidst these dire events, Qiu approaches the Soul Keeper to sacrifice his own future for Chun's safety, but the price is steep—his entire lifespan. Amidst a labyrinth of choices, he learns how to counteract the impending doom. As tensions escalate, the community unites to eliminate Kun, leading Qiu to attempt a portal that could save both worlds. However, Kun struggles to navigate the passage as calamities unleash torrents of sea water into Chun's world.
Feeling the weight of her choices, Chun faces the community’s disdain, even from her mother. In a poignant moment of selflessness, she merges with her grandfather's begonia tree to seal the breaches, thwarting the flood and protecting her people.
In a twist of fate, Kun takes a branch from the begonia and presents it to the Soul Keeper, who resurrects Chun, sending her on a journey to reunite Kun with his home. As they share one last evening together, Qiu is paralyzed by regret, unable to voice his true feelings.
The following day, as Chun bids a heartfelt farewell to Kun, Qiu opens one final portal. Unbeknownst to Chun, he casts a spell that will send her through while ensuring his demise. As he sacrifices himself, he whispers that he will forever be with her. Transformed once more into a red dolphin, Chun follows Kun into the human realm.
On the shore of the human world, they awaken in their human forms. As Kun stirs and sees Chun reaching out to him, their destinies intertwine once again. In a haunting mid-credits reveal, the Soul Keeper resurrects Qiu from the ashes, appointing him as her successor, unveiling the profound price of love and sacrifice in this enchanting tale.
