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. Unbeknownst to Nagasumi, mermaid law dictates that if a human discovers their existence, both the human and the mermaid must be executed unless they marry. To save both their lives, Sun’s family, the Seto Group—a powerful yakuza clan of merfolk—forces Nagasumi into an engagement with Sun. The series revolves around Nagasumi’s attempts to navigate his new life, balancing his relationship with Sun while dealing with the chaotic and often violent antics of her family.

Sun Seto is a kind-hearted and devoted mermaid who genuinely cares for Nagasumi, though her naivety and over-the-top reactions often lead to comedic situations. Her father, Gozaburo Seto, is the ruthless leader of the Seto Group, fiercely protective of his daughter and initially hostile toward Nagasumi. Sun’s mother, Ren Seto, is more level-headed but equally formidable, often mediating between her husband’s extreme actions and Nagasumi’s well-being. The Seto Group’s enforcers, including the stoic Masa and the eccentric Lunar, frequently interfere in Nagasumi’s life, creating additional challenges.

Nagasumi’s ordinary life is further complicated by the arrival of other mermaids, such as Mawari Zenigata, a childhood friend of Sun who harbors feelings for Nagasumi, and her loyal shark companion, Edomae. The series also introduces rival mermaid families, including the Ikkaku and Saru groups, who add to the chaos with their own schemes and conflicts. Amidst the comedic and action-packed scenarios, Nagasumi strives to maintain his relationship with Sun while surviving the unpredictable world of merfolk yakuza.

The narrative blends slapstick humor, romantic tension, and absurd situations, often escalating into over-the-top confrontations involving exaggerated martial arts, underwater battles, and elaborate schemes. Despite the constant turmoil, the core of the story remains the growing bond between Nagasumi and Sun, as they navigate their unconventional relationship and the challenges posed by their respective worlds. The series balances its comedic tone with moments of genuine emotion, exploring themes of loyalty, family, and love amidst its chaotic and fantastical premise.