Kyou Sogoru, an average high school student and avid swimmer, resides in the scenic Maihama City. His routine consists of spending time with friends, swimming, and playing video games. However, his ordinary existence is upended when Shizuno Misaki, a beautiful and enigmatic girl, makes an unusual request: that they jump into their school's pool together. This decisive plunge transports Kyou to a war-ravaged world where humans pilot formidable humanoid mechs called Zegas to battle hostile aliens known as Gards-Orm. To lead this effort, humans have established Cerebrum, a rebel faction dedicated to stopping the Gards-Orm from wiping out humanity. As Kyou engages in combat missions, interacts with other Zega pilots, and observes numerous fatalities, he starts to doubt the true nature of this world and his own existence.