自律機動戦車イヅナ

The protagonist Asakura Akino, after she lost her parents she lives together with her older brother.
Her brother Souichirou goes overseas to study in America, and so it's decided that Akino will be taken in by her aunt's family.
Among the oppositions of some relatives Akino sympathizes with the feelings of her traveling brother. She sends him off with a smile, however.
Even now there is something she thinks about. At that time, maybe I should have told him my true feelings.

This story starts a few years after that. For the sake of Souichirou's livelihood Akino and him move to the rural town Tsugawa.
Akino transfers into the place where her brother works, Hakojitsu Techno Vally Academy, there she reunites with her childhood friends, Sugita Munechika and Takayanagi Kou.
For some reason the two of them are conducting some secret research along with her brother, but this information isn't revealed to Akino.
However on a certain day, by an unexpected coincidence Akino learns about the existence of their development of the "Autonomous Mobile Vehicle Izuna".

(Translated from the official site)