Long ago, Japan and its people were at war against the vile Demon Race. The demons were destroyed, but the blood of its leader was preserved and history was repeated. Humanity now fights this reemerged threat by forming the United Nations of Humankind. Y/N is a soldier for the UNH to fight back against the demons but was raised and trained by a demon seeking to stop the bloodshed. During a mission, he meets a girl who is part of the secret Demon Slayer Corps. Y/N is encouraged by his guardian to join them and kill the one leading the Demons. The battle for the world is drawing near. Will the bonds formed be enough to change the fate of humanity?