"Merman in My Tub" centers on Wakasa, a young man who discovers a merman named Tatsumi in a river near his home. After rescuing Tatsumi from a precarious situation, Wakasa brings him home, where Tatsumi decides to live in Wakasa’s bathtub. The series explores their daily lives as they navigate the challenges of coexisting, blending mundane human routines with the fantastical elements of Tatsumi’s merman nature.

Tatsumi is a carefree and somewhat mischievous character, often displaying childlike curiosity about human life. His presence introduces Wakasa to a world of aquatic creatures, including other merfolk and sea spirits, who occasionally visit or cause minor disruptions. Despite his initial reluctance, Wakasa grows accustomed to Tatsumi’s antics and develops a sense of responsibility for his well-being. Their relationship evolves into a unique friendship, marked by humor and occasional moments of heartfelt connection.

The narrative is episodic, with each chapter focusing on small, self-contained stories that highlight the interactions between Wakasa, Tatsumi, and their expanding circle of friends. These include other merfolk like the reserved and elegant Mikuni, the energetic and competitive Takasu, and the mysterious sea spirit Toki. The series balances lighthearted comedy with subtle explorations of themes such as belonging, acceptance, and the blending of different worlds.

The setting primarily revolves around Wakasa’s home, particularly his bathroom, which serves as Tatsumi’s primary habitat. However, occasional excursions to rivers, beaches, and underwater realms provide variety and expand the scope of the story. The manga maintains a relaxed, slice-of-life tone, with occasional bursts of whimsy and fantasy that underscore the contrast between the ordinary and the extraordinary.

Character dynamics drive the plot, with Wakasa’s grounded, practical personality often clashing with Tatsumi’s impulsive and playful nature. Despite their differences, their bond deepens over time, offering moments of warmth and mutual understanding. The series avoids heavy drama, instead focusing on the charm of its characters and the gentle humor derived from their interactions. Through its simple yet engaging premise, "Merman in My Tub" presents a lighthearted and imaginative take on interspecies friendship and the quirks of everyday life.