A high school boy named Keisuke Sakudaira is suddenly summoned to another world just before he enters high school, where he's told he must defeat the Demon King as a hero. However, the sacred weapon (Holy Armament) bestowed upon him is nothing more than his ordinary school bag...!? Soon after, Keisuke is labeled as a defective hero and is abandoned by the queen, his summoner, in the most dangerous dungeon filled with powerful monsters. It seems he is doomed to die... But then, the "useless weapon" — the school bag — is revealed to have amazing cheat abilities that will change everything! Thus begins the unbelievable journey of a hero traveling through another world armed with nothing but a single bag!

"If you let us stay... you can do naughty things with us." Mitsuhiko Okaya, an unremarkable man, was betrayed by his wife and a junior colleague. At work, he was mocked as a useless editor and spiraled into despair, until one day before him appeared a pair of twin high school girls he shouldn't know, yet somehow seemed familiar. The younger twin, Akari, was lively, cheerful, and always teasing him, while the elder twin, Nanako, was shy and clumsy, yet secretly a closet pervert. They were once his students from the cram school where he worked during his student days.

Gamma Snipe used to take down enemies from super long distances with his incredible range. However, because his distances were too long, he was kicked out by his party leader, who didn't understand his work. Then, a former female classmate visits him, saying, "I've come to scout you." She works in the private army of the princess of a neighboring country. Gamma joined the army on her recommendation, and with his incredible range, he takes on important missions one after another, becoming an indispensable member of the army. Meanwhile, his former party, having lost their valuable fighting force, continues to fail quests...

In this world, when people turn 15, they are granted a skill from the heavens. Hirak Mattoh was born into a family of Sword Saints and was hailed as a prodigy from a young age. Everyone was certain he would inherit the same title. However, at his skill appraisal ceremony, he received Unlocker — a seemingly worthless ability that could only "open what is closed." Disappointed, his family branded it a trash skill and exiles him. Left adrift, Hirak soon discovered its true power. By unlocking his status window, he could freely acquire skills, obtain item drops, and grow stronger without limit.

Ryuuji who was an orphan was picked up and raised by the world's strongest evil dragon. When he turned 15, he had intended to leave the house and be independent, but his mother proclaims that to do so he must defeat her. "You're lvl 999! How the hell am I supposed to do that?!" Somehow, he manages to persuade her and he is allowed to journey out, however, she ends up joining him as his companion. "You wish to join my son's party? Then you'll have to pass the interview with me." "An extermination quest? It's dangerous, so let your mother handle this." "Dungeon exploration? I've already cleaned up the small fry there." Due to the overprotective mother by his side, suffering and misfortune continue to pour. Will he be able to stand on his own as an adventurer and as a man?