The time is during Crystal Tokyo.  The senshi, save Pluto, have all died.  Neo-Queen Serenity's last wish was that Usagi, now 15, be able to train as a senshi with others.  She founded a school for that.  Only problem was, the school was right in the middle of the enemy base, undetected, and it's current students were already in the middle of a war.  How will Usagi get to the school, or better yet, how will she help with the war?