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).

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