In a nutshell, it can be summarized in one sentence. The three time travelers carrying the Onmyoji, Yume Shu and Touken Ranbu systems have forgotten what happened before the world was reset, but their partners still remember it.
The world has been reset, this is an unchangeable fact.
However, the three people who are at the center of the world reset do not remember this. They only remember them as companions.
They are on the very high school-level street where they attend Hope's Peak Academy.
But at the same time, Hyuga Sou, Naegi Makoto, and the final one all have secrets they don't know. This is the key and result of the world reset.
The timeline after the world is reset is before the biggest and most evil desperate event in history occurs.
Purpose: Socialist core values