There is such a rule in the mermaid world. If a human accidentally sees the real body, then there is no third choice except marrying him or killing him. The ordinary boy Manchao Nagatsumi (voiced by Mizushima Daisuke) was rescued by the mermaid Seto Can (voiced by Taoi Haruko) in a drowning. According to the rules, Can became Nagatsumi's wife. Unexpectedly, Can turned out to be the daughter of the underworld boss of the Seto Sea. What kind of tragic fate will Nagatsumi, who has a cowardly personality, face? At the same time, as Nagatsumi and Can's wedding date progresses, more and more mermaids gather around Nagatsumi. The dual personality of Edo Mae Runa (voiced by Nogawa Sakura), the one who wants to kill Nagatsumi (voiced by Kuwatani Natsuko), the shrewd and capable swordsman Shiranui Mingno (voiced by Kitamura Eri), these lovely mermaids not only add a lot of trouble to Nagatsumi's life, but also bring a lot of happiness.