魔物が跋扈する『ダンジョン』が世界各地に出現して数年。社畜リーマンの水樹了介は、職場がダンジョンに飲み込まれた現場に居合わせ、チートスキル『スキルブック』を手にする。これを機に冴えない社畜人生から一転、未知なる冒険へ挑むことになるのだが……!? It has been several years since “dungeon”, in which monsters lie, has appeared all over the world. Ryosuke Mizuki, a company slave Lehman, is present at the scene where the workplace was swallowed by the dungeon and gets the cheat skill “Skill Book”. Taking this opportunity, I will change from a dull company slave life and challenge an unknown adventure …!?