Mangetsu Kohinata is an ordinary school girl. The only unusual thing she seems to have is the routine of making bento for her classmates, however whether her "friends" really appreciate it or just take advantage of her is debatable. One day she forgets a lunch box at school and comes to the classroom for it. On the way back, Mangetsu is suddenly drawn into an alternate dimension where giant magical mechs brutally fight each other. After escaping the initial aftermath, the apparently average girl gains the ability to control her own mech - the White Lily. From here on, she has to participate in this mysterious, raging battle, and try her best to survive, no matter if she desires it or not.