The story begins with the protagonist, a young man named Lect, being stripped of his "Explorer" license and his title as the "Dungeon Slayer" by a nobleman in the royal castle of the Westit Kingdom. Lect, who had single-handedly destroyed seven S-rank dungeons, is informed that his services are no longer needed. The kingdom's policy has shifted from destroying dungeons to coexisting with them, viewing them as sources of valuable resources rather than mere monster nests. Lect argues that coexistence is impossible, as dungeons are living entities that will exploit any leniency, but his pleas are dismissed by the nobleman who calls him a mere mercenary with no other skills. Lect is also forced to return his signature black mask, which was provided by the kingdom, revealing his unremarkable face and identity as Lect, a commoner.
After leaving the castle, Lect, surprisingly unfazed by his disgrace, wanders through town. He encounters Misha, a twelve-year-old girl with striking red hair and a precocious intelligence, who is the granddaughter of the Explorer's Association Chairman. Misha is furious about Lect's expulsion, having learned of it through her grandfather. She reveals that the mask was a tool for the nobles to discard Lect when he was no longer useful, allowing them to frame him as a fake if he ever rebelled. Lect acknowledges this, mentioning that only the Princess was ever truly on his side. Misha, concerned for Lect, urges him to rely on her instead of the Princess, warning that the Princess might "imprison" him. Lect, however, states he isn't in dire straits and could work in other countries, which momentarily distresses Misha before he reassures her he intends to stay and repay his debts to the kingdom. He expresses a desire to retire from dungeon exploration, feeling that destroying dungeons endlessly is futile and that a new era is dawning. Misha then proposes that Lect become an instructor at the Dungeon Training School, where he once studied.
Suddenly, an emergency alert interrupts their conversation: students from the training school are in distress in the D-rank dungeon "Swirling Abyss." A rescue team, the B-rank party "Blue Sword Training," has been dispatched, but the situation has worsened, with the rescue targets suffering severe injuries. Lect, despite no longer being an official Explorer, volunteers to go, stating he has ample funds and can act as a volunteer. He rushes to the dungeon, arriving at the sixth floor where the rescue targets, a young woman and a girl, are found. Lect quickly assesses their condition and prepares to bring them back to the surface.
Meanwhile, the "Blue Sword Training" party, led by the A-rank Explorer Medi, is struggling. They encounter an unusually large number of monsters and a Minotaur, a B-rank monster not typically found in a D-rank dungeon. Despite their efforts and using a shortcut, they are overwhelmed. Just as Medi is about to be defeated, Lect intervenes, effortlessly defeating the Minotaur and destroying the axe. He then quickly rescues Medi and the injured girl, bringing them to the surface to meet Misha and the association staff. Misha notes Lect's immense power, warning him not to unleash his full strength lest he break the continent.
Later, Lect begins his new career as an instructor at the Dungeon Training School. He meets the school director, Karina, a former instructor of his, who is aware of his true identity. Karina acknowledges Lect's decision to retire and offers her support. She assigns him a class of students, noting their high elemental levels and the emergence of a "golden generation." She tasks him with humbling any arrogant students. During his first practical lesson, Lect is impressed by the students' knowledge but also notes their overconfidence. He demonstrates his own limitations, being able to use only the basic "Elemental Clad" technique, which leads some students to underestimate him. He then observes the students practicing their elemental abilities, noting their impressive skills, particularly two-attribute "Elemental Clad" users, Ray and Smelk, and the powerful earth-user Shach. He also notices Alice, the girl he rescued, struggling with elemental manipulation. The lesson ends with a chaotic incident where a student's uncontrolled wind elemental attack causes a rock to fall, endangering Hal.
In a separate scene, two men, one of whom is identified as a Duke, discuss their failed attempt to kill the Duke's daughter in the "Swirling Abyss" dungeon, implying they used a magical "antenna" embedded in the monsters. They are frustrated by the intervention of a high-ranking Explorer and plan their next move.
