The protagonist, a perpetually overworked and underpaid office worker named Shunsuke Kasamatsu, is forced by his supervisor to retrieve medicinal herbs from the Toyoshima Labyrinth after a mistake in ordering. Despite having no prior experience with labyrinths, Shunsuke reluctantly agrees, hoping to finish quickly and enjoy his rare Sunday off.
Upon arriving at the Toyoshima Labyrinth, Shunsuke finds the entrance unattended and the "closed" signs absent, leading him to believe it's permissible to enter. He descends into the labyrinth, which is initially illuminated by an emerald green glow and features a straightforward path. However, the expected landmarks and monsters fail to appear, and the path seems to extend far beyond the initial 200 meters. Exhausted and frustrated, he notices a faint red light ahead and decides to investigate before returning.
The red light emanates from a narrow gap in a rockfall at a dead end. Driven by curiosity and a desire for some reward, Shunsuke manages to push the large rock aside, revealing a scorching hot cavern filled with lava. As he cautiously peers into the abyss, the ground beneath him crumbles, and he narrowly avoids falling into the lava. Suddenly, a colossal Fire Dragon emerges from the molten sea. In a panic, Shunsuke attempts to close the rockfall, but his actions cause the rock to tilt and fall, trapping him. He flees, pursued by the dragon's breath and explosions.
Miraculously, Shunsuke survives the onslaught, collapsing from exhaustion. He then receives a series of system notifications, indicating he has achieved a "Giant Killing" by defeating the Fire Dragon, earning him the titles "Fool" and "Dragon Slayer," along with skills like "Spatial Storage" and "Equivalent Exchange." He also levels up to Level 4. The system automatically collects the dragon's drops.
Upon returning home, Shunsuke discovers that the labyrinth's layout and monster placement change on full moon nights, explaining his disorientation. He also learns that his acquired skills are initially unusable outside the labyrinth due to legal restrictions, but can be activated by using skill points. He uses his "Equivalent Exchange" skill to convert some of the dragon's drops into skill points, allowing him to unlock "Restriction Release," which permits the use of his skills outside the labyrinth.
Shunsuke then researches the value of the herbs he was sent to collect, discovering they are worth a substantial amount, enough to significantly improve his financial situation. He also realizes the immense value of the dragon's drops, particularly the "Fragment of the Fire Dragon's Scale," which he sells at a Shibuya branch of the Labyrinth Management Bureau for two million yen. This substantial sum solidifies his decision to quit his job and pursue a new life as a "Diver," someone who explores labyrinths. He registers as a Diver at the Toyoshima branch and receives a "Labyrinth Pass," a magical item that functions as a debit card for his labyrinth earnings.
