Nagasumi Michishio, a middle school student, finds himself in a life-threatening situation during a summer vacation at Seto Inland Sea. While swimming, he nearly drowns but is saved by a mermaid named Sun Seto. However, mermaid law dictates that if a human discovers their existence, both the human and the mermaid must be executed. To circumvent this, Sun’s family proposes a solution: Nagasumi must marry Sun, thereby making him part of their world and exempt from execution. Reluctantly, Nagasumi agrees, and Sun moves in with his family.

Sun Seto is the daughter of a yakuza-like mermaid family, the Seto Group, which wields significant influence in the underwater world. Her father, Gouzaburou Seto, is a formidable and overprotective figure who initially opposes the marriage but eventually accepts it under the condition that Nagasumi proves himself worthy. Sun’s mother, Ren Seto, is more supportive but equally eccentric. The series explores the comedic and chaotic dynamics of Nagasumi’s new life as he navigates the challenges of being engaged to a mermaid while dealing with her family’s antics.

The narrative is driven by a mix of slapstick humor, romantic tension, and absurd situations. Nagasumi’s attempts to adapt to his new reality are frequently interrupted by Sun’s family, rival mermaid gangs, and other supernatural entities. A recurring antagonist is Masa, a shark-like mermaid assassin sent by a rival group to eliminate Nagasumi, though her efforts are often comically inept. Another key character is Lunar Edomae, a mermaid from a rival family who develops feelings for Nagasumi, adding further complexity to the romantic subplot.

Throughout the series, Nagasumi’s relationship with Sun evolves from one of obligation to genuine affection, though their bond is constantly tested by external forces. The story balances over-the-top comedy with moments of heartfelt emotion, particularly in episodes that delve into Sun’s insecurities about her identity and Nagasumi’s growing resolve to protect her. The series also incorporates elements of Japanese folklore and mythology, blending them with modern-day settings and character archetypes.

The supporting cast includes Nagasumi’s human friends, such as his best friend Chimp, who often provides comic relief, and his classmate Mawari, who harbors a crush on him. These characters add depth to the human side of the story, creating a contrast with the fantastical elements of the mermaid world. The series concludes with Nagasumi and Sun reaffirming their commitment to each other, though their future remains uncertain due to the ongoing challenges posed by their respective worlds.