The protagonist, Atobe Arito, finds himself in a long, dark tunnel, part of a procession of people. He realizes he was on a company ski trip and must have died in a bus accident. He is guided by a woman who explains that their souls have been transferred to a new world called "Labyrinth Country" for reincarnation. Upon arrival, he receives a sturdy card and is instructed to go to the Guild to determine his profession.
In Labyrinth Country, life revolves around exploring labyrinths. Arito learns that about thirty people from his company also reincarnated. He heads to the Guild, where he witnesses a senior adventurer warning a newcomer about the dangers of even low-level labyrinths. Arito considers forming a party with his former colleagues, but many are quickly absorbed into existing groups. He sees Suzuna Shiromiya, a girl he noticed on the bus, register as a "Miko" (Shrine Maiden). Shortly after, a renowned silver-haired swordswoman named Elithea Centrail, despite her own difficulties, recruits Suzuna into her party.
Arito then proceeds to register his own profession. He is informed by the Guild receptionist, Luisa Falmer, that professions are determined by aptitude. He observes a large, boisterous man being forcibly removed for demanding a specific "Hero" class, highlighting the importance of following the Guild's rules and the concept of "Karma." Arito, a former advertising planner, struggles to choose a profession, lacking any particularly special skills beyond basic English. He eventually decides to write "Rearguard" on his license, a vague term that surprises Luisa. She explains that this is an unprecedented profession and provides him with "Mercenary Tickets" to hire non-human help, specifically a Lizardman, to help him understand his new role.
Arito hires a female Lizardman named Theresia, who is a "Rogue" class, a versatile mid-range fighter. He learns that "Demi-humans" are humans transformed by labyrinths and that their "Subjugation Seals" allow them to be controlled. He also learns about the "License" system, which tracks experience, health, magic, and skills. Arito chooses "Support Defense 1" and "Support Attack 1" as his initial skills.
Their first foray into the "Field of Dawn" labyrinth proves challenging. The weakest monsters, "Cotton-like Creatures," are surprisingly powerful. Arito's "Support Defense 1" skill proves effective in mitigating damage to Theresia, and his "Support Attack 1" skill, when combined with Theresia's attacks, allows them to defeat a "Red Face," a named monster that had previously annihilated another party, including Arito's former boss, Kyoka Igarashi. Arito's actions earn him experience, leveling him up and increasing his "Trust" with both Theresia and Kyoka, who he manages to aid with "Support Recovery 1" after she is injured. He learns that "Support Defense" can affect multiple party members and that "Support Attack" adds damage.
After returning from the labyrinth, Arito learns that he can potentially permanently hire Theresia by paying a substantial fee. He also encounters his former colleagues, Suzuna and Elithea, and his boss, Kyoka, who is recovering. He discusses the dangers of "Named Monsters" with a rescue-specialist adventurer named Rival. Arito's actions in defeating the Red Face are kept confidential by Luisa, who is impressed by his unique profession. He resolves to gather more party members and improve his skills to survive in Labyrinth Country.
