Narrated by the protagonist Keiji Kiriya, the story unfolds as he joins the United Defense Force to combat the enigmatic beings known as 'Mimics' invading Earth. Despite meeting his demise during his initial mission, Keiji inexplicably finds himself reliving the day before the battle, trapped in a relentless cycle of death and rebirth. With each iteration, Keiji hones his combat prowess, endeavoring to alter his destiny within the time loop.