During his summer break, Nagasumi Michishio, a middle schooler, embarks on a trip to the Seto Inland Sea. While swimming at Mio Sun Beach one day, he experiences a sudden leg cramp, leading him to the brink of drowning unnoticed. In a stroke of fate, a mermaid appears just in time, rescuing him from the depths. That evening, Nagasumi meets his savior, Sun Seto, a mermaid hailing from a yakuza lineage. Learning the grave consequences of a mermaid exposing her identity to a human, Nagasumi faces a dire ultimatum from Sun's powerful father: marry Sun or face a deadly fate. Forced into matrimony to escape death, Nagasumi and Sun must navigate the challenges of concealing their union. Amidst constant threats on Nagasumi's life and the interference of various foes, a pure and heartfelt bond blossoms between the unlikely couple as they strive to adapt to their peculiar circumstances.