In the small town of Evergreen, an ordinary man named Alex finds himself transformed into a tiny black fish named Fin after a night of drowning his sorrows in alcohol. Awakening in an enchanting isekai world, Fin is driven by an insatiable curiosity, unlike his fellow fish who are content with their mundane lives.

Determined to seek answers to the questions burning in his mind, Fin embarks on an adventurous journey. Leaving the safety of his small pond, he navigates the river, encountering both beauty and danger along the way. He marvels at the mesmerizing aquatic landscapes, meets fascinating creatures, and faces the ominous Water Serpent that haunts the river's heart.

Through trials and tribulations, Fin's bravery and determination grow, and he gains wisdom from the sage-like catfish, Kuro. With each passing day, his desire to explore the vast ocean intensifies, and he remains undeterred even as he faces treacherous currents and whirlpools.