During his summer vacation at the Seto Inland Sea, middle school student Nagasumi Michishio finds himself in danger while swimming at Mio Sun Beach. A sudden leg cramp prevents him from staying afloat, and with no one nearby to hear his cries for help, he begins to drown. Just as he loses consciousness, a mermaid rescues him. Later that evening, his savior reveals herself to be Sun Seto, a member of a mermaid yakuza family. Under mermaid law, if a human learns a mermaid's true identity, the mermaid is sentenced to death. To avoid this outcome, the Seto family offers a choice: Michishio must marry Sun or face execution by her father and clan leader, Gouzaburou. With no other option, Michishio agrees to the marriage. As husband and wife, they must keep their relationship hidden. Amid constant threats on Michishio's life from Gouzaburou and the strange behavior of other enemies, genuine affection begins to grow between the pair as they adjust to their unusual life together.

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