The story begins with Ciel, an eighteen-year-old Dungeon Fairy who appears sixteen, meticulously preparing for a crucial boss summoning. Despite her perfect academic record at Dungeon Academy, excelling in subjects like Monster Lore and Dungeon Strategy, Ciel is plagued by anxiety. Dungeon Fairies are tasked with summoning a boss monster, to whom they must then serve and help grow their dungeon. The summoning process is entirely random, and there's a risk of being eaten by the summoned monster or, if the monster refuses to become a boss, the Dungeon Fairy enters a ten-year hibernation. Ciel, having mastered monster languages and communication, feels she has done all she can.
The summoning ritual commences with a room bathed in blue light and ancient, untranslatable runes appearing on the walls, which Ciel interprets as a blessing. However, her hopes are dashed when the summoned monster is revealed to be a slime, the weakest of all monsters, incapable of communication or becoming a boss. Ciel is devastated, lamenting her perpetual bad luck.
Suddenly, the slime speaks in human language, asking where it is. This slime, who will become the protagonist, wakes up with amnesia, initially believing he's hungover. He discovers his lack of physical features like eyes, nose, mouth, and limbs, and his gelatinous blue body. Ciel, shocked by his ability to speak and his slime form, reveals he is the boss monster she summoned. The protagonist, deducing he was once human and now a slime, jokingly suggests he's a hero transformed by a demon king, but Ciel corrects him. He then demonstrates his slime abilities, including a dissolving liquid, much to Ciel's dismay and embarrassment as it melts her clothes.
After Ciel introduces herself as a Dungeon Fairy scouting him as a boss, the protagonist agrees to the contract, albeit with a condition of being named "Bastard" (nicknamed "Tard"). He learns that Dungeon Bosses have three main duties: recruitment, expansion, and defense. Initially, his only available recruit is baby slimes, which he finds pathetic. He also learns about dungeon points, earned by defeating intruders, which can be used for expansion and growth. Ciel, using her appraisal magic, reveals her own skills and his status as a slime with "Magic Immunity," which, to her dismay, also negates her support and healing magic, making him effectively a magic-proof slime.
The protagonist and Ciel then venture into the Chaos Labyrinth, a different world filled with wild monsters. Ciel, despite her fear, is tasked with scouting and translating. The protagonist, using his knowledge of human behavior and a baby slime as bait, successfully traps a goblin in a pit. However, his attempts to recruit the goblin fail, and he resorts to a humiliating threat involving dissolving liquid and a suggestive message, which the goblin rejects before being rescued by its kin.
Ciel then identifies seven other monster types in the area, including Killer Ants. The protagonist decides to recruit Killer Ants, believing their strict hierarchy under a Queen Ant makes them reliable. They locate an ant nest and, using a baby slime as a gift, Ciel negotiates an audience with the Queen Ant. The Queen, initially wary of a slime boss, agrees to lend two Killer Ants for a week in exchange for baby slimes, sensing the protagonist's ambition and the potential for future growth. The protagonist reveals his true intention: to use the Killer Ants to eventually take over the Queen's nest, a risky strategy he calls "crossing a dangerous bridge."
A week later, the dungeon has expanded, and the protagonist has used points to create jam-filled bread, which Ciel tearfully cherishes. They discover two Killer Ants are now helping with dungeon expansion, acting as intruders who generate points. The protagonist also learns about the "Wind Ghost Committee," a faction of Dungeon Fairies who oppose dungeons that harm humans, and the dangers of human interaction, like taxes and wars. Ciel creates a face for the protagonist using paper, giving him eyes and a mouth, which he finds more fitting than his original slime form.
The Killer Ants discover a cave leading to a bandit hideout. The protagonist, realizing the danger and the potential for points, decides to eliminate the bandits. He also learns about the "Chaos Town," a hub for Dungeon Fairies where they can buy items and hire mercenaries. Ciel hesitates to reveal this information, fearing judgment for her choice of a slime boss.
The protagonist and Ciel then seek a cursed sword, Murasame, located in a deep ravine. They overcome the challenge of descending the ravine using Ciel's levitation magic and the protagonist's magic immunity. Upon finding the sword, they encounter a beautiful woman, Murasame, who is a manifestation of the cursed sword. She warns them away, but the protagonist, recognizing her true nature and desire to cut, offers to be her master, allowing her to fulfill her purpose. Murasame, realizing her curse stems from her own desires, accepts, and the protagonist claims her as his subordinate, much to Ciel's surprise.
In the following week, Murasame, now a subordinate, provides ear-cleaning services to the protagonist, while Ciel monitors the bandits. The bandits, former mercenaries, are poorly equipped and generate significant points. Murasame, a sentient cursed sword, is added to the monster management screen. The bandits plan to rob a church carriage, which poses a significant threat as the church opposes dungeons and could lead to their destruction. The protagonist decides to eliminate the bandits and uses his points to gather information about communication methods between Dungeon Fairies, learning about the Chaos Town. He plans to go there to acquire more powerful allies.
