The setting is modern-day Japan. One day, a massive direct-hit earthquake strikes the Kanto region, revealing the entrance to a vast labyrinth stretching deep underground. From the monsters infesting the labyrinth, people discovered they could harvest not only various rare metals but also a precious, previously unknown substance called "Mana." Consequently, volunteer civilians began challenging the labyrinth as Explorers to subjugate monsters and collect Mana.

【The Labyrinth】 The entrance at the Shibuya Scramble Crossing is the most famous, but several others exist. The structure can change without warning. It contains diverse natural environments, ranging from stone-built corridors to giant natural caves lush with greenery. Currently, six sectors (Sectors 1 through 6) have been identified, but what lies beyond remains unexplored territory. The boundaries between sectors are marked by "Gates" made of a marble-like material, serving as milestones for progress. The further a sector is from the entrance, the higher its number and the greater the difficulty of exploration. While the Explorer Association provides guidelines, the Labyrinth is effectively a lawless zone where bloody incidents, including murders between Explorers, occasionally occur. Mana obtained in the Labyrinth is collected at the exit gates and converted into cash. A novice Explorer can earn about 50,000 yen worth of Mana in a single day; without injury or death, an annual income can exceed 10 million yen.

【Explorer Association】 Also known as the Labyrinth Guild. A state-run organization responsible for managing and operating Labyrinth-related facilities and the Explorers who challenge them.

【Armament】 Most Explorers are issued registered melee weapons by the Association upon entry. Weapons are collected when leaving the Labyrinth. Firearms are discouraged due to risks like ricochets, though exceptions may be granted for special joint public-private exploration units.

【Secondary Rewards】 Equipment brought in by other adventurers or weapons of unknown origin can be found within the Labyrinth; these become the property of whoever recovers them. Explorers often give these found weapons names like "Muramasa" or "Tonbogiri," though the name does not always match the performance.

【Death in the Labyrinth】 Humans who die in the Labyrinth have their bodies coated in Mana, which stops decomposition, entering a state known as a "Mummy." Mummified corpses can sometimes be revived using the Explorer Association's resurrection facilities, but this costs at least 10 million yen. It is said that 50% of Explorers who challenge the Labyrinth experience their first death within one month.

【Supernatural Powers】 An unknown life energy derived from "Mana" permeates the Labyrinth, allowing for the use of abilities commonly referred to as psychic powers or magic. Activation methods vary—chanting spells, using catalysts like cards or powders, meditating with closed eyes, or dancing—and the effects are equally diverse.

【Labyrinth Creatures】 These include mutated flora and fauna, demi-humans, and more. To date, no individuals capable of communicating with humans have been discovered, and most are hostile toward humans.