Mitis Nagasumi high school student arrives for the summer to his grandmother, whose house stands on the shore of the Gulf of Seto. While bathing he brings his leg, and the poor man starts to sink. Nagasumi probably would have died if not for the intervention of a young mermaid... San - chan makes a teenager, but now Nagasumi, and San,and their families are facing a difficult choice: because no one must know of the existence of mermaids, or San -chan, or Nagasumi will perish. However, there is another option: San Nagasumi and can get married, then their family pedigree, and which may be a secret from relatives..?Poor Nagasumi! Easy Does it - to get married a year before graduating from high school, and even at the mermaid underwater yakuza clan..!