"This Boy Caught a Merman" is a short anime series that blends fantasy and slice-of-life elements. The story centers around Shima, a reserved and introverted high school student who lives alone in a seaside town. One day, while fishing, Shima accidentally catches a merman named Wakasa. Wakasa is cheerful, curious, and somewhat mischievous, contrasting sharply with Shima's quiet demeanor. Despite initial reluctance, Shima allows Wakasa to stay in his home, leading to an unusual cohabitation.

As the story progresses, the relationship between Shima and Wakasa evolves. Wakasa’s presence gradually draws Shima out of his shell, encouraging him to engage more with the world around him. The series explores themes of loneliness, connection, and personal growth, as Shima begins to confront his own emotional barriers. Wakasa, though seemingly carefree, also reveals moments of vulnerability, hinting at a deeper backstory tied to his existence as a merman.

The narrative is punctuated by lighthearted moments and subtle emotional undertones, creating a balance between whimsy and introspection. Supporting characters, such as Shima’s classmates and townsfolk, add depth to the setting, though the focus remains primarily on the dynamic between Shima and Wakasa. The series concludes on a reflective note, leaving the future of their relationship open to interpretation while emphasizing the impact they have had on each other’s lives.