Nagasumi Michishio, a high school student, nearly drowns during a summer vacation trip to the Seto Inland Sea but is saved by a mermaid named Sun Seto. According to mermaid law, if a human discovers their existence, both the human and the mermaid must be executed unless they marry. To save Nagasumi’s life, Sun’s family, the Seto Group—a powerful yakuza-like organization of merfolk—forces the two into an arranged marriage. Sun moves in with Nagasumi, and their lives become increasingly chaotic as they navigate the challenges of their sudden union.

The Seto Group, led by Sun’s overprotective father Gozaburo and her mother Ren, is fiercely protective of Sun and often intervenes in her relationship with Nagasumi. Gozaburo, in particular, is prone to extreme and comedic reactions, frequently threatening Nagasumi with violence for perceived slights against Sun. Despite the constant interference, Nagasumi and Sun gradually develop genuine feelings for each other, though their relationship is complicated by misunderstandings and external pressures.

The series introduces a colorful cast of supporting characters, including Maki, Sun’s childhood friend and a skilled martial artist who harbors unrequited feelings for her; Luna, a rival mermaid from the rival San Group who also develops feelings for Nagasumi; and Nagasumi’s classmates, who become entangled in the chaotic events surrounding the Seto Group. The narrative is driven by a mix of romantic comedy, slapstick humor, and absurd situations, often involving over-the-top confrontations, misunderstandings, and the clash between human and mermaid cultures.

As the story progresses, the focus shifts between Nagasumi and Sun’s evolving relationship, the antics of the Seto Group, and the various challenges posed by rival factions and external threats. The series balances its comedic elements with moments of heartfelt emotion, particularly in exploring the bond between Nagasumi and Sun as they grow closer despite the chaos surrounding them. The interplay between the human and mermaid worlds serves as a backdrop for the series’ exploration of themes such as loyalty, family, and the complexities of young love.