Minato Wadatsumi, despite being born to a long line of fishermen and having lived in a port town for his entire life, is crippled with fear of fish and unable to swim. However, his life takes an unexpected turn when one day, while returning a stranded fish to the sea, he is swept away by a massive wave and finds himself in the underwater world of Latimeria, surrounded by enchanting mermaids.
He soon discovers that according to Claire, the Queen of the Deep Sea, and known as Primo Mermaid, he has been appointed as Latimeria's prince. Moreover, with a kiss of true love, he possesses the special ability to convert a mermaid into the next queen. Despite his initial reluctance, Minato soon realizes that the only way for him to return to the surface is to acquire the power of the Primo Mermaid, thus leading him to accept his newfound role as prince.
As his caretaker, Akari Irie, helps him become acquainted with the mermaids, he begins to see them as regular girls. However, he discovers that they are capable of transforming into deep-sea creatures as well. Despite many girls pursuing the position of Primo Mermaid, Minato's fear of fish prevents him from accepting the candidates for the role.
