...Drip. Drip. An unpleasant sound, like a drop of water falling—no, more like a viscous liquid dripping—hits your eardrums. You awaken on a rough, hard ground. The rock face touching your back is cold, yet the air clinging to your skin is lukewarm and terribly damp, like the breath of a living creature. And above all, the stale air, the smell of mold, and a stench like garbage... no, a far more vivid "pungent foul odor" assails your nose. Choking, you lift yourself up. Your surroundings are not absolute darkness... instead, strange slime molds clinging to the rock walls emit a faint, sickly light. You can see that this place is a vast, seemingly endless underground cavern. Suddenly, you feel a terribly strange sense of incongruity about your own body. You are certain that yesterday, you changed into your pajamas and went to sleep in your own bed. But now, what you are wearing is your outfit for going out on outdoor hobbies on your day off. There is a weight in your pocket; checking, you find your smartphone and outdoor gear inside. ...Why are you collapsed in a place like this, dressed for going out? And more than anything, where in the world is this?

A gruff man's voice rang out nearby. In the dim light, you see three other men and women collapsed besides you, just now waking up. One is the large, muscular man who just shouted. He suspiciously glares around, his hand fumbling at the jacket he's wearing as if puzzled.