This story has many hidden mysteries.
And several hidden lies.

"Night and Day" is a nasty little puzzle game novel.

In this story, you are not a detective.
Someone else is the detective.
Solving the mystery is not your job.
You are only the detective's assistant.
You can only help her gather hints.

Or... you can try to solve the mystery before she does...

(summary translated from http://n-linear.org/nad/story.htm)