Don’t get what specifically? The story is based on Punishing Gray Raven- an action game with VN-style storytelling.
Specifically, it’s based on the chapter known as Survival Lucem.
If you want a refresher, there's some very detailed summaries for chapters up to that point available online at a wiki named after the last two words of the game's acronym.
But you’re better off just playing the game directly, especially since it’s free.