The story begins with Sakudaira Keisuke, a 15-year-old ordinary student, being unexpectedly summoned to another world along with three other Japanese individuals. They find themselves in a ritualistic setting with a magic circle, summoned by Queen Walujo von Geeta Niiga of the Geeta Niiga Kingdom. The Queen explains that a Demon Lord is threatening their world, and local forces are no match for him. She implores the summoned individuals, whom she calls "Heroes," to defeat the Demon Lord.
Keisuke, unlike the others who express confusion or excitement, feels a thrill of anticipation, having grown up reading light novels and manga. The other summoned individuals are Yamada Charao, a seemingly frivolous young man; Ueda Shizuka, a quiet woman; and Iida Otaku, an enthusiastic otaku. Charao initially expresses a desire to return home, but the Queen reveals that while summoning is possible, sending them back is not, unless the Demon Lord knows the ancient magic.
The Heroes are then informed they will receive the "Three Divine Artifacts": an Item Box, Appraisal skill, and a Holy Weapon unique to each Hero, which they are bound to equip. Keisuke, Charao, and Shizuka receive a sword, a spear, and a bow respectively. To Keisuke's dismay, his Holy Weapon is revealed to be a school bag, leading to ridicule from Charao and Shizuka, though Otaku tries to offer encouragement. Upon using Appraisal, Keisuke discovers his "Hero's Bag" possesses the "Box" skill, which is identical to the Item Box skill, making him feel like he received a duplicate ability.
Later, Keisuke is summoned by Queen Walujo, who reveals he is deemed "unnecessary" due to his seemingly useless Holy Weapon and is to be "disposed of" in the "Seven Hells," a dungeon filled with monsters. She justifies this by stating that direct execution would tarnish the royal family's prestige. Stranded in the dark, glowing cave, Keisuke despairs, realizing he only has Appraisal and Item Box skills. He is then attacked by an S-rank monster, a Viper. In a moment of desperation, he instinctively uses his bag to store the Viper's corrosive poison, which causes his Holy Weapon to level up and gain a new skill: "Fetch Bag," allowing him to retrieve items from anywhere. He then discovers he can also store the Viper itself, further leveling up his bag and gaining the "Monster Box" skill. He also finds he can retrieve modern Japanese items like water and snacks, realizing his bag might be more powerful than he initially thought.
Keisuke explores the dungeon, using his skills to survive. He encounters the skeletal remains of another Hero, Toyooka Mobuo, who was also apparently discarded. By storing Mobuo's remains, Keisuke gains several new skills, including Mapping, Stealth, Lockpicking, and Dismantling. He also acquires Mobuo's 300,000 gold. He learns that the dungeon has 250 floors and that his bag's abilities seem to grow stronger when he stores powerful entities or items.
He then encounters a powerful Fenrir, Spellvia, who is sealed in the dungeon. Spellvia, initially hostile, is appeased by Keisuke's snacks and agrees to become his friend. Keisuke learns that Spellvia was sealed by a previous Hero, and that the dungeon is the lowest level. He discovers that storing Spellvia, along with her sealing barrier, "Absolute Barrier," causes his bag to level up significantly, gaining new skills like "Magic Box" and "Hakoniwa" (a personal pocket dimension).
With Spellvia's help, Keisuke begins to ascend the dungeon. He learns that his bag's abilities are exceptionally powerful, and that the skills from the discarded Heroes he stores are also accessible to him. He also discovers that his cooking skills, enhanced by his bag and the "Hero's Pot," can grant buffs, which inadvertently caused Spellvia to grow to an immense size and blast a hole through the dungeon ceiling. This allows them to bypass many floors by flying.
They encounter other discarded Heroes' remains, collecting their skills and items. Keisuke also learns that his bag's abilities are linked to the number of Holy Weapons he possesses, accelerating its leveling. They face various monsters, with Keisuke using his growing arsenal of skills and Spellvia's guidance and power. He also discovers that his cooking can grant significant stat boosts, which Spellvia experiences firsthand.
Eventually, they reach a floor where they find another Hero, Misaka Ai, trapped in a glowing box by a curse. Misaka, a "Great Hero of Divine Eyes," was sealed by the royal family for her past achievements, as she was deemed inconvenient after bringing peace. Keisuke, moved by her plight, uses his bag to free her and her sealing artifact. He also learns that his cooking skills can even break through supposedly indestructible dungeon walls, and that his bag's "Hakoniwa" skill allows them to create a safe zone.
