The year is 2025. Aliens invade Earth, triggered by the arrival of a comet. To counter the sudden invasion, humanity develops the anti-alien weapon known as the “Synthetic Human” (or “Android”), but ultimately, they are wiped out.

Time passes, and we find Rihito, a Synthetic Human living alone in a desolate world where humanity is extinct. By chance, a high school girl named Seina, who is from the past, time-slips to his era, and the two begin a journey together.

Rihito gains incredible strength and levels up by defeating aliens. As they travel, the two are drawn to each other and fight to prevent the future of human extinction. However, during this struggle, Seina confronts a single possibility:

—If they change the future, Rihito might disappear.