In the distant future (c. 6000 AD), Humans have spread across the Galaxy and have split into two factions; the Galactic Union and the Deague, which are currently at war. To allow for practical interstellar travel, both factions have developed sub-lightspeed drives, which allow ships to travel huge distances in a greatly reduced subjective time (due to Time Dilation).
The Union’s strongest force is made up of mecha called Strains, short for Strategic Armored Infantry, operated by pilots called “Reasoners”. In order to become a Reasoner, a potential pilot must have a Mimic, a machine fused with brain cells taken from the Reasoner before birth – thus, if the mimic is damaged or destroyed, it cannot be replaced. The Deague do not possess Strain technology, but instead utilize swarms of small automated attack fighters called Tumors.
Sara Werec, an orphan, is the focus of the story. As the series starts, she despairs that her older brother Ralph, who is considered the best Reasoner in the Galactic Union, has been assigned a mission that will take him 130 lightyears away to the front lines; due to time dilation, even if he does survive, Sara will have died of old age long before he could ever return home. She decides that when she grows up, she will become a reasoner herself and join the war in order to see her brother again.