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In the world of mermaids, there is a rule: if a human accidentally sees a mermaid's true form, the mermaid has only two options—either marry the human or kill them. Ordinary teenager Manami Nagare (voiced by Daiyu Mizushima) is saved from drowning by the mermaid Seta Aki (voiced by Haruko Tachibana). According to the rule, Aki becomes Nagare's wife. However, Aki turns out to be the daughter of the leader of the underworld in Seta Bay. What tragic fate will the timid Nagare face? Meanwhile, as the wedding between Nagare and Aki approaches, more and more mermaids gather around Nagare. These include Edomae Rana (voiced by Sakura Nogawa), who has a split personality; Kajiri (voiced by Natsuko Kugaya), who is determined to kill Nagare; and the sharp and capable swordsman Shikou Meino (voiced by Eri Kitamura). These adorable mermaids not only bring a lot of trouble to Nagare's life but also a great deal of happiness.