The story is told from the perspective of Keiji Kiriya, the protagonist, a new recruit in the United Defense Force which fights against the mysterious 'Mimics' which have laid siege to Earth. Keiji is killed on his first sortie, but through some inexplicable phenomenon wakes up having returned to the day before the battle, only to find himself caught in a time loop as his death and resurrection repeats time and time again. Keiji's skill as a soldier grows as he passes through each time loop in a desperate attempt to change his fate.