The setting is modern-day Japan. One day, a massive earthquake strikes the Kanto region, revealing the entrance to a vast Great Labyrinth stretching deep underground. From the monsters nesting within, 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 the monsters and collect Mana.

【The Labyrinth】 The entrance at Shibuya's 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 mazes to massive natural caverns lush with greenery. Currently, six sectors (Sectors 1 through 6) have been identified, but what lies beyond remains unexplored territory. "Gates" made of a marble-like material stand at the boundaries between sectors, serving as milestones for progress. The further a sector is from the entrance, the higher its number and the greater the difficulty. While the Exploration Association provides guidelines, the Labyrinth is effectively a lawless zone where bloody incidents, including killings between Explorers, occasionally occur. Mana obtained inside is collected at the exit gate and converted into cash. A rookie Explorer can earn Mana worth roughly 50,000 yen in a single day; barring hospitalization or death, an annual income can exceed 10 million yen.

【Labyrinth Exploration 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 generally discouraged due to the risk of ricochets, though exceptions may be granted for special joint public-private exploration units.

【Sector Difficulty Guide】 It is generally said that three months of experience are needed to progress through one sector. Reaching Sector 4 requires nine months (3 months * (4-1)), and only after a full year of experience can one finally reach Sector 5. Explorers who reach Sector 5 account for only about 0.1% of the total population.

【Secondary Rewards】 Equipment brought in by other adventurers or armaments of unknown origin can sometimes 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 necessarily match the performance.

【Death within the Labyrinth】 Humans who die in the Labyrinth have their bodies coated in Mana, which stops decay, entering a state known as a "Mummy." Mummified corpses can sometimes be revived using the Exploration 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 a month.

【Supernatural Powers】 An unknown life energy derived from "Mana" permeates the Labyrinth, allowing for the use of abilities commonly referred to as superpowers 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, as well as demi-humans. So far, no individuals capable of communicating with humans have been discovered, and most are hostile toward people.

【Outside the Labyrinth】 Outside the Labyrinth lies the cityscape of Shibuya. While the number of Explorers and people in related industries has increased since the Labyrinth appeared, little else has changed, and the streets remain lined with the many restaurants and shops found in modern Japan.