The story is told from the first person point of view of the protagonist Keiji Kiriya. Keiji is a new recruit in the United Defense Force, fighting against the mysterious creatures called "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 skills as a soldier grows as he passes through the time loops trying to change his fate.