On a full moon night, Subaru - a merman - coincidentally saved a man who sank into the ocean. After that, in order to see that man again, Subaru traded off his tail for a pair of legs and came to the surface to find Tatsuki - the man's name. Despite of his throbbing feet and his lost voice, Subaru was happy to be able to stay by Tatsuki's side. However, his happiness soon faded away when he found out the reason why Tatsuki was at the bottom of the ocean that day.
Note: A sequel to Mister Mermaid, "Mister Mermaid After That," was published in doujinshi.