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 until a near-drowning in the ocean changes everything. The girl who rescues him, San, seems perfect at first. Then she reveals the truth: she's a mermaid, and in her world's brutal code, humans who learn the secret must either die or marry into the family. There's no middle ground. With both their lives on the line, Nagasumi makes a desperate proposal. He agrees to marry San and become part of her yakuza clan, which means living a double life. By day, he's a regular high schooler. By night, he navigates an underwater criminal empire full of trigger-happy fish-men, overprotective in-laws, and supernatural obstacles that make normal relationship drama look quaint. What starts as a survival strategy becomes unexpectedly genuine. Nagasumi and San's forced marriage develops real affection, but keeping her existence secret while staying alive tests them constantly. Someone's always hunting for proof that mermaids exist, and her family keeps throwing increasingly dangerous situations at them both. This is chaos wrapped in genuine warmth, where love blooms amid constant absurdity.