To protect his older brother, the younger brother chose to fight— "Tokyo Mutation" A major disaster caused by monsters overflowing from a mysterious vortex that suddenly appeared in the Tokyo sky. Although it caused many casualties, the situation was brought under control by Awakened ones who awakened Skills to fight the monsters. Four years since the "Tokyo Mutation." Yuu Unai, who awakened an S Rank Skill, had been spending all his time conquering Dungeons to ensure that no harm from monsters would reach his older brother, his only family. One day, while conquering a Dungeon as usual, he receives news that a Dungeon has appeared at his brother's school――