Shirogane Takeru, an ordinary Japanese teenager, was transported to an Earth under attack by alien creatures called BETA. Like any would-be hero, he volunteers to save the world, only to fail miserably in the attempt. Now he's stuck in a time loop, and forced to watch again and again as his friends die and he fails to complete his mission. After countless iterations, though, it's finally time for things to change... Takeru's only hope, and humanity's, is for him to track down his former teacher Kouzuki Yuuko, and ensure the completion of the mysterious Alternative IV project. But how can he do that when he doesn't even know what it is?