Arata Narue Sekai is a one episode ova that is one part of a three part story involving time travel (The ova takes place in the future, the manga in the past, and the light novel in the present). The ova has not been translated into English and I have only read the manga, so some of this might be inaccurate.
The series takes place in the past, present, and future where humanities collapse will, is about to, or already has happened.
Yakusa is the protagonist of the ova and was sent to the future, where the same day endlessly repeats, with with three other girls, no instructions on what to do, and only a toy like cell phone that can contact a fellow time traveler named Kirishima who was sent to the past. It seems to two were split up to prevent them from meeting as their descendants are key to the invention of time travel and that time travel itself maybe the root cause of humanities collapse.
