According to an ancient legend, mermaid's flesh can grant immortality if eaten. 500 years ago, Yuuta 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. Yuuta kills the mermaids and rescues the girl, but she has already eaten some of the mermaid's flesh. Although he had to kill the mermaids, Yuuta isn't too disappointed because Yuuta'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, Yuuta and Mana attempt to seek out more mermaids, trying to become normal humans again.

Source: ANN

Note: The story of 1991 OVA Ningyo no Mori (Mermaid Forest) is re-told in episodes 4-5 and the story of 1993 OVA Ningyo no Kizu (Mermaid's Scar) is re-told in episodes 12-13.