Sixteen-year-old Yukito Kirihara cares about his 14-year-old sister Sana more than anyone. When an ogre appears outside his window searching for her, he jumps in to protect her and is killed. A mysterious woman then offers him reincarnation and hands him a character sheet inside a strange book. Spotting Sana in that book as well, Yukito sets out to find her and save them both, despite having just 1 HP to work with. As the title suggests, he aims to clear the dungeon as fast as possible in an action fantasy adventure.