In the distant future, humanity is split between two factions, Union and Deague, locked in an endless war whose origins have long been forgotten. The Union relies on the Strain, a manned weapons system, as the core of its military, while the Deague deploys the unmanned Tumoru. Sara Werec, a 16-year-old from a respected family, attends the Grapera Space Armed Soldier Academy in the Cranial Solar System, training to become a full-fledged Reasoner and Strain pilot. Her brother, Ralph Werec, was dispatched on a special mission to the frontlines 130 light-years away when she was 11. While the journey takes 400 years in objective time for the fastest Union vessels, the theory of relativity means only two subjective years pass for those aboard. Having lost their parents at a young age, the siblings share a deep bond, and Sara's greatest desire is to reunite with Ralph by traveling to his battlefield. Suddenly, Deague forces assault Sara's planet. To her shock, Ralph appears among the raiders, successfully kidnapping a mysterious girl found sleeping in a capsule. Determined to uncover the truth, Sara pilots a Strain to pursue him. However, Ralph attacks her, resulting in damage to the Mimic, a copy of the pilot's brain that is essential for controlling the Strain.