This Boy Caught a Merman

Tatsumi is a high school boy who lives by himself. One day, a devastatingly handsome merman named Wakasa moves into the bathtub in Tatsumi's home. Wakasa is a bit self-centered but cute, while Tatsumi is cool but a busybody.

Michishio Nagasumi's life couldn't be any more normal. In an odd twist of events, during his summer vacation he ends up almost drowning in the sea. Luckily, the cute mermaid Seto San appears to save him. However, San is from a yakuza mermaid family and according to their law, if a human is to catch sight of a mermaid, either he or the mermaid must die. The only other way is for Nagasumi to be taken in as a family member, marrying San. In attempts to save both of their lives, Nagasumi asks for San's hand in marriage. Nagasumi's summer vacation reaped more then what he would expect, as he must now protect San from others finding her secret out.

Legend has it that when you eat the flesh of a mermaid, you would live forever. Yuta and Mana are living proof of this... Together, they journey to various places trying to find meaning for their existence, or perhaps even a "cure" for their situation. One day, Yuta and Mana meet Masato, a little boy who seems terrified of his mother Misa. It seems to Yuta and Mana that mother and son have a very unusual relationship. What happens when Yuta and Mana decide to discover the dark secret Masato and Misa are hiding? And what about those monster-like creatures that have been appearing?

According to an ancient legend, mermaid's flesh can grant immortality if eaten. 500 years ago, Yuta unknowingly ate a piece of mermaid's flesh. For centuries, he travels across Japan, hoping to find a mermaid, thinking she may be able to make him a normal human again. When he finally finds one, he discovers that she and her companions have been raising a girl to be their food so they can eat her and take on her youthful looks. That is how mermaids stay young. Yuta kills the mermaids and rescues her, but she has already eaten some of the mermaid's flesh. Although he had to kill the mermaids, Yuta isn't too disappointed. Yuta's once lonely existence is now over, as he has found a companion in Mana. And Mana, who had been trapped in a small hut her whole life, finds delight in even the simplest of things. Together, Yuta and Mana attempt to seek out more mermaids, trying to become normal humans again.