The story follows Suzuki Toru, a high school student who, despite being in an environment ideal for making friends, finds himself without any human companions. His only true friend is Shizuku, a white slime who can shapeshift into any form, most often adopting the appearance of his favorite idol, Kokone Miru, for convenience in their adventures. Toru's motivation for exploring dungeons, which are accessed through a hidden door in his dorm room's closet, is to gain experience points to level up, which he believes will improve his academic and athletic abilities, and potentially help him make friends.
Toru lives in Koharu-so, a dilapidated dorm managed by his grandmother, Chiharu. He took over the management role as a condition for attending the private Daiichi Academy. The dorm is rumored to be haunted and is largely avoided by outsiders.
One day, Toru discovers that the closet in his room leads not to his futon, but to a stone-walled space with an iron door, revealing a dungeon. He encounters blue slimes, which he defeats with a metal bat, experiencing a surge of strength and realizing he has leveled up. This discovery fuels his desire to level up further, hoping it will improve his social standing and help him make friends.
He learns from Shizuku, who reveals herself to be a white slime with a history, that the dungeon is called "Yomi's Dungeon" and is located in the southwestern part of the Francis Kingdom. Shizuku explains that white slimes were once symbiotic with humans, taking on desired forms and serving them, but were later exploited and forced to hide in dungeons. She chose to ascend from the dungeon to find a kind human, and found Toru. She can also transform into any person and even mimic their abilities, having scanned Toru and later the student council president, Rokunomiya Himeko, to learn about human forms and abilities.
During an attempt to scan Himeko, Shizuku inadvertently causes her to faint, leading Toru to cover for her. He also learns that the dungeon entrance in his room disappears at certain times, preventing him from accessing it. He begins to suspect that other dorm residents might also have access to dungeons. He meets his fellow dorm residents: Kono Kotaro, a mushroom enthusiast who cooks mushroom dishes daily, and Miyu Reiko, a mysterious first-year student who is always hidden behind her long black hair and has apparently repeated a year.
Toru's goal shifts from simply leveling up to understanding the world of dungeons and magic, and potentially finding a way to meet people from the Francis Kingdom. He also grapples with the ethical implications of Shizuku's abilities, particularly her scanning of humans. The chapters end with Toru and Shizuku preparing for school, with Shizuku disguised as Toru's uniform, and Toru contemplating the possibility of other students having similar hidden abilities.
