At the age of eight, Aragaki Kuya was involved in a shipwreck and was transported to another world. The other world seemed tranquil at first glance, but it was a world where the countdown to its destruction had begun. It was a world where men suddenly turned into monsters called Remmas! Kuya was at a loss when his master, who had guided and raised him as a child, also became a Remma. Then, a ship carrying a girl from the neighboring continent washed ashore in front of him――!? The only man left in the world can change the world on the brink of destruction!