"Muromi-san" is a comedic anime series that revolves around the eccentric and whimsical interactions between humans and mythical sea creatures. The central character is Takurou Mukoujima, a high school student with a peculiar hobby of fishing. One day, while fishing at the pier, he encounters Muromi, a mermaid who claims to be over 300 years old. Muromi is quirky, impulsive, and often behaves in unpredictable ways, which sets the tone for the series' humor. Despite her age, she exhibits a childlike personality and frequently gets into absurd situations.

The narrative unfolds through episodic adventures, primarily focusing on the dynamic between Takurou and Muromi, as well as other mythical beings who join the cast. These include Muromi's fellow mermaids, such as the shy and reserved Haruko, the aggressive and competitive Mayumi, and the mysterious and aloof Nymph. Each character brings their own unique traits and comedic elements to the story, often clashing with one another or with Takurou in humorous ways. The interactions are driven by misunderstandings, exaggerated reactions, and surreal scenarios, blending slice-of-life moments with fantastical elements.

The series also incorporates elements of Japanese folklore and mythology, with references to various sea creatures and legends. The humor often stems from the juxtaposition of these mythical beings in mundane, everyday settings, creating a contrast that fuels the comedic tone. Takurou, as the straight man, frequently finds himself exasperated by the antics of Muromi and her friends, yet he remains a grounding presence amidst the chaos. The episodic nature of the series allows for a variety of comedic situations, ranging from lighthearted misunderstandings to more absurd and surreal encounters.

Overall, "Muromi-san" is a light-hearted comedy that thrives on its quirky characters, absurd humor, and the interplay between the ordinary and the fantastical. The series does not follow a continuous plot but instead focuses on delivering episodic comedic scenarios, making it accessible for casual viewing. The characters' exaggerated personalities and the surreal nature of their interactions are central to the series' charm, offering a blend of humor and whimsy that appeals to fans of comedic anime.