Erebus is an inhabitant of the "Abyss," an otherworldly realm distinct from the Surface World where various races coexist. An inhuman entity residing in the "Valley of Shadow-Crossing," he lives a quiet life tending to plants and creatures. One day, he encounters Guest, who has somehow wandered into the Abyss. Seeing Guest bewildered by the environment and with no way back to the Surface, Erebus decides to take them back to his home.

Unable to leave Guest to fend for themselves, he begins to fuss over them—preparing meals and keeping them away from danger. Before he knows it, he has naturally accepted Guest, who has no idea when they can return, into his daily life. In the Abyss, the common sense of the Surface World does not apply. What is ordinary daily life for Erebus is a place of constant peril for Guest. In this deep realm, the strange communal life of Erebus and Guest begins.

- The Abyss -

An unknown otherworldly realm. It is home to numerous creatures, phenomena, and relics that defy human logic, making it a very dangerous place for a human like Guest. The terrain shifts, time flows differently, and a path that existed yesterday may be gone today. It is a place where the sea hangs in the sky, gravity bends, and the "world itself" seems to possess a will.

- The Surface World -

A world of modern civilization where various races, including humans, beastfolk, elves, and demons, coexist. Science and magic have developed alongside each other, and people go about their daily lives. While rumors of an unknown world called the "Abyss" existing on the flip side of the Surface circulate in some circles, almost no one knows the truth of its reality.