Everything in Psycho-Sleuth happens in one place, which is the point. A gothic cathedral has been seized by the Rational Order (理教), and thirteen people have been summoned into it. The cathedral is the locked room, the cult is the pressure, and the twenty-hour script is what happens inside.
The ritual the cult proposes is a murder without being caught — one of the thirteen kills, the killing is never solved, and the survivor becomes god. The twist the game puts on the table early is that the cult leader is one of the thirteen. So the selection is rigged by construction, and every one of the thirteen has a reason to solve it rather than participate in it.
The thirteen are called the Diseased (病能者). Each carries a real cognitive disorder, and each disorder grants a power of madness (病能) — a concrete mechanic that distorts the player's own view of the scene. The disorders are not flavour text; they are the game's systems. A character with hemispatial neglect literally hides half the screen from you. A character with prosopagnosia turns corpses faceless. These are the puzzle's vocabulary.
You play as Lain (萊爾), who carries synesthesia and the power of World Creation (世界創造). That power lets him make other people see his hallucinations. It also means the narrator of the investigation is a person whose perception is known to be unreliable — and who is himself the protagonist. The game keeps that ambiguity live rather than resolving it early, which is the single strongest structural decision in the script.
The madness powers are described as contagious, and Chapter 2 makes that literal: the contagion adds symptoms to other characters, which adds new lines of dialogue, which changes what the evidence means. A disorder that was one character's problem in Chapter 1 becomes a shared condition that reshapes the investigation. That is a well-constructed way to make a "what is actually happening" question into a mechanic.