History rewrites itself around Kai when the hero Sid, the man who won humanity's war against four rival races, is erased as if he had never existed, and nobody but Kai notices the change. In this new timeline, humanity lost, Kai himself was never born, and he's a stranger even to people who once knew him. After a mysterious girl named Rinne finds him, Kai sets out to become the hero the world forgot it needed. This manga adapts the light novel's opening arc.