In the year 2025, aliens invaded Earth, their arrival heralded by a passing comet. To counter the sudden invasion, humanity develops the anti-alien weapon "Artificial Humans," but they were ultimately annihilated— Time flows onward. In a desolate world long after humanity's extinction, an Artificial Human named Rihito lived in solitude. His lonely existence being alone suddenly changed when Seina, a high school girl from the past, accidentally time-slips to his era, and the two embark on a journey together. By defeating aliens, Rihito was able to level up, displaying astonishing power. As the two of them grow closer to each other, they struggle to change the future of humanity's extinction. However, in the midst of their fight, Seina is confronted by a terrible possibility. —If she changes the future, Rihito might cease to exist. A sci-fi fantasy about two people from across time weaving their own destiny!