Kosuke Inoue, a 30-year-old adventurer with the lowest rank, joins a team led by Mizuki, a girl who is a candidate for a heroic role, just to make up the numbers. Contrary to their expectations, Mizuki and the others are astonished by Inoue's unparalleled performance due to his accumulated experience! In fact, Inoue is a "legendary adventurer" who has survived the worst of the worst dungeons! Mizuki recognizes Inoue's ability and asks him to teach her how to fight as an instructor - a tale of an adventurer specializing in survival and training an S-class high school girl to be the strongest hero!