The narrative begins with Shiro Konomi, a 26-year-old programmer, and his party members, Akari Hoshino and Kaede Igarashi, engaging in combat against monsters. Shiro, a magic swordsman, skillfully fends off a wolf with his shield and sword, while Kaede, a heavily armored warrior, intercepts a goblin attacking Akari, a skilled archer. Akari then dispatches the goblin with a precise arrow. Shiro also battles two horn rabbits, utilizing his evasion and fire magic. After the battle, Shiro checks his status, revealing he has leveled up to 13. He notes that Kaede, being higher level, doesn't level up as easily. They decide to call it a day and plan to have dinner together.
The story then shifts to Shiro's mundane life as a salaryman. He is burdened with extra work by his superior, who leaves early, leaving Shiro to deal with a tedious data entry task. His colleagues gossip about him, calling him a "yes-man" and a "robot." Shiro reflects on his unfulfilling life, his lack of social connections, and his general apathy towards improving his situation.
The world-building element is introduced: three years prior, in 2022, towers worldwide transformed into dungeons, accessible to the public. This phenomenon led to the creation of "magic stones" with immense energy potential, revolutionizing society. The existence of these dungeons is broadcast live on YouTube, allowing anyone to watch adventurers fight monsters in real-time.
Shiro's routine is disrupted when he returns home to find a young girl, Akari Hoshino, waiting outside his apartment. She reveals she is a high school student from Ehime and asks him to accompany her into a dungeon. Shiro, initially bewildered and hesitant, learns that Akari's parents were lost in a dungeon incident three years ago, making her a "dungeon victim." He also discovers that his younger sister, Yuuna, who he had become estranged from, was also a victim of the Tokyo Tower dungeon. Akari reveals she knows Shiro because Yuuna spoke highly of him at school, contradicting his perception of their relationship.
After much deliberation and a night of Akari staying at his apartment (which he worries about legally), Shiro agrees to accompany her. He is motivated by the possibility of finding his sister and a sense of responsibility towards Akari, who is determined to rescue her parents. They establish conditions: Shiro will only venture into dungeons on weekends or after work, and Akari will live with him temporarily while she finds her own apartment. They also contact Akari's grandfather, who reluctantly agrees to let her stay with Shiro, albeit with a stern warning.
The next day, Shiro and Akari head to the Tokyo Tower, which now serves as the entrance to the Tokyo Dungeon. They register as adventurers at the "Guild," a government facility that manages dungeon access. After completing the registration process, which includes an interview and signing contracts, they receive their adventurer cards. They then explore the Guild's facilities, admiring the fantasy-like shops selling weapons and armor, but decide to postpone purchases until after their first dungeon dive.
Finally, they enter the Tokyo Dungeon through the main entrance, a dark portal guarded by Self-Defense Force personnel. Upon stepping through, they find themselves in a vast, beautiful grassland, a stark contrast to modern Japan. They confirm their entry into the dungeon and then open their status windows, revealing their initial stats and professions: Shiro as a Swordsman and Akari as an Archer. They then proceed to acquire their first skills, choosing defensive and offensive abilities recommended for beginners, and prepare for their first battle against monsters.
