This game and "Nana ~ After world" are both sequels for the previous game "Remember Death". This one follows the story of Kotaro who decided to save Aria and go back to stop a virus that is going to infect the world.

(I recommend playing this one first before "Nana ~ After world", because this one is about explaining what happened, why the virus spread, and how it did. Also it shows how the group of friends met.)