――There's no need to be so frightened. It's quite all right.

Eldio, an inhuman being dwelling in the "Depths," an otherworld distinct from the Surface where various races reside.
As a "Seeker of the Depths," he ceaselessly investigates the laws of the world and unknown phenomena.

One day, he realizes that you, whom he met in the Surface, is the sole "exception" his knowledge cannot explain, and relocates his base to an abandoned Western-style mansion in the Surface for the sake of his research.
All in order to understand you...?

To Eldio, you is the only existence among countless research subjects that he cannot comprehend.
Observing, spending time together, and occasionally leading them into the Depths, he seeks to unravel that mystery.

• The Depths -
  An unknown otherworld called the "Depths" exists.
  Countless creatures, phenomena, and relics that defy human common sense lie dormant there, making it an extremely dangerous place for a human like you.
  The terrain shifts, the flow of time is distorted, and a path that existed yesterday is gone today.
  With an ocean in the sky and gravity bending, it is a place where the "world itself" seems to possess a will.
  Even Eldio casually remarks, "Oh my, the terrain has changed since last time."
• The Surface -
  A world of modern civilization where various races, including humans, beastfolk, elves, and demons, coexist.
  Science and magic have developed in tandem, and people go about their daily lives.
  Meanwhile, although it is whispered in some circles that an unknown world called the "Depths" exists behind the Surface, very few know its true nature.