This is the first chapter in the game.
This walkthrough is copied from vgperson's.
Leave Misane's room and enter Nanase's (the middle door).
Talk to Nanase. Leave the room and talk to the girl in the hall. Head right to the lobby and talk to the man.
Take the elevator to 2F: Gimmick Room. Return to the lobby and take the elevator to 4F: Garden. Return to the lobby, then left to the rooms, and a scene will trigger in the hall. Head right to the lobby, and a scene will trigger.
Talk to the mechanic in front of the elevator. Take the elevator to 2F: Gimmick Room. Go to Nanase and select the magnifying glass. Check all the points in the scene, then return.
Go down to the lobby and talk to Teruhi, Koza, and Merue. After talking to Koza, also use the magnifying glass to show him the Little Door. After getting info from everyone, interrogations will begin.
1. Teruhi At statement 1 ("searchin' for something"), present Extension Cord.
2. Koza At statement 3 ("hurried to look for Teruhi"), present Eerie Voice. Show Fatcats.
3. Merue At statement 4 ("why did you die in that room"), ask more, and point out the issue. Show Shitsuna's Actions.
4. Teruhi 2 At statement 2 ("you're gonna put up some resistance"), present Sleeping Pills. Show Shitsuna's Actions. Show Cat Door.
5. Merue 2 At statement 4 ("I don't even have the power"), ask more, and say there was some means. Show Gimmick Room.
6. Koza 2
At statement 4 ("is the method you propose not impossible in practice?"), present System Issue.
Lastly, present Victim.