In the distant future, mankind is divided into two powers: Union and Deague. They have continued a war for so long that no one is sure when it began. Union uses manned weapons, mecha called Strain, while Deague uses unmanned weapons, the Tumoru.
The 16-year-old Sara Viereck, is the daughter of a established Union family, who goes to Grapera Space Armed Soldier Academy located in Cranial Solar System in order to become a full-fledged Reasoner (a pilot of Strain).
Her brother, Ralph Viereck, was sent to the front line 130 light years away to accomplish a special mission when she was 11 years old. It takes 400 years of objective time to travel that distance — even by the fastest sub-light speed Union space craft. However, according to relativistic theory, it only corresponds to two subjective years (the time the crew experiences). Therefore, Sara needs to go to the same battle field to meet her brother.
Since they lost their parents when they were quite young, Sara thinks of her bother as an individual dearer than a sibling, and she lives for their reunion.
One day, the planet Sara presently resides on is suddenly assaulted by Deague. Surprisingly, it is her brother Ralph who steals a mysterious girl sleeping in a capsule.
To search for the truth, Sara gets in Strain and follows him. However, she is attacked by Ralph, and in the process the Mimic — a "copy" of the pilot’s brain, an essential part required to control the Strain — is severely damaged.