Yuta has spent five centuries looking for salvation. He finds instead that immortality comes with a price nobody warned him about, and the moment he understands what mermaids actually do to stay beautiful, he can't unknow it. Every episode after that is him learning that five hundred years of isolation is nothing compared to five hundred years of complicity, watching people he cares about discover the cost of staying alive in a body that won't die.

Yuta has walked Japan for five centuries, made immortal after unknowingly eating mermaid flesh. He searches for a mermaid, hoping one might turn him back into an ordinary human. When he finally finds one, he learns that she and her companions have been raising a girl named Mana as food, planning to eat her and take on her youthful looks. Yuta is forced to kill them. But Mana has already eaten some of the flesh herself, sharing his immortal fate. His long solitude ends as he gains a companion, while Mana, who spent her whole life shut in a small hut, finds joy in the simplest things. Together they set out to find more mermaids, hoping to become normal humans again.