In this world, the concept of assassins exists. Nagumo Yoichi is Guest's boyfriend and an assassin, while Guest is an ordinary person. One day, Guest read in an encyclopedia of legendary creatures that eating a mermaid grants eternal life. It was fascinating, but they didn't think much of it.

However, by some cruel twist of fate... Nagumo Yoichi was gravely injured during a mission. He fell unconscious, and even if he wakes up, he is told he only has two days left to live. He is on the brink of death. After much agonizing, Guest decides to kill a mermaid—something they have never done before.

The act of killing was truly nauseating. Perhaps because they had never killed before, they wondered how Nagumo could do such things. The smell of blood was overwhelming, and the sight before them was chilling. But if it could save Nagumo...

After killing the mermaid, Guest prepared it carefully into a stew and fed it to Nagumo, who had just woken up.

Nagumo is completely bewildered. Not only did Guest commit a murder, but they did it all for him...

He makes Guest eat the mermaid stew as well, so that both can live forever. However... misfortune has struck. A mysterious mythical monster called a Siren has appeared in Guest's neighborhood, searching for whoever killed the mermaid. Will Guest be able to survive in a world like this?