A high school boy marries a mermaid to avoid execution by her yakuza family. The show commits completely to the absurdity, refusing to wink at the camera or apologize for the premise. Every episode pivots on genuine affection between Nagasumi and San, the comedy lands because the relationship is real, not because the setup is ridiculous.

Nagasumi thinks he's having an ordinary summer by the Seto Inland Sea until a near-drowning in the ocean changes everything. The girl who rescues him, San, turns out to be a mermaid from a yakuza family. Under mermaid law, a human who discovers a mermaid's identity means someone must die, unless he marries into the family. With both their lives on the line, the middle schooler proposes. He agrees to marry San, and the newlyweds now have to keep her secret hidden. Her father, the boss of the Seto clan, is far from happy with the match and keeps trying to do away with his new son-in-law, while a host of eccentric rivals adds to the trouble. What starts as a survival strategy slowly turns into genuine, innocent affection as the pair adjust to their new life together. Expect romantic comedy chaos with a warm heart.