Kyou Sogoru, an ordinary high school student and an ardent swimmer residing in the charming Maihama City, enjoys spending time with his pals, swimming, and playing video games. However, his regular routine is thrown into a chaos when Shizuno Misaki, a stunning and enigmatic girl, makes an odd proposal to him - to jump into the school's pool with her. Little did Kyou know that this jump would transport him to a world of chaos and destruction, where humans battle against vindictive aliens, the Gards-Orm, with the help of state-of-the-art humanoid robots called Zegas. The human race has formed a rebel group, Cerebrum, to spearhead the war and thwart the Gards-Orm's mission of wiping out humanity. As Kyou embarks on numerous combat missions, acquaints himself with other Zega pilots, and witnesses countless casualties, he begins to doubt the authenticity of this world and question the purpose of his own existence.