Nagasumi Michishio, a teenage boy, is saved by the mermaid Sun Seto while drowning in the Seto Inland Sea. Despite his parents not believing it, all three are taken by the mermaids where they meet Sun's family. Mermaid laws are incredibly strict and state that Sun must die for allowing Nagasumi to see her mermaid form. Sun's father, Gozaburo Seto, along with most of his yakuza members, prefer to solve the problem by killing Nagasumi. Sun's mother, Ren Seto, suggests that Nagasumi simply marry Sun, since married couples are allowed to know the mermaid secret, thereby avoiding having to kill Sun or Nagasumi. Desperate to avoid being murdered by Sun's father, Nagasumi formally asks for Sun's hand in marriage.