When Earth is overrun by deadly Mimic creatures, humanity bands together to form the United Defense Force, a collective effort to combat this menacing threat. Equipped with advanced exoskeleton suits, soldiers engage in a relentless battle against the invaders. Amidst this chaos, newcomer Keiji Kiriya meets a sudden demise on his first mission, only to awaken the day before the fateful event. Caught in a time loop upon each death, Keiji seizes the opportunity to unravel the mysteries of his predicament. Through countless cycles of the same harrowing day, he hones his skills and knowledge, determined to alter his inevitable outcome and tip the scales in humanity's favor.