The story begins with an unnamed university student's late-night encounter with a three-meter-tall, one-eyed monster resembling a Cyclops. Just as the creature is about to kill him, a "magical girl" named Satsuki, dressed in a Taisho Roman-style red hakama, appears and incinerates the monster with fire magic. The protagonist, fearing memory erasure or worse, hides from Satsuki, throwing 5000 yen at her as a clumsy thank you before escaping into the river, catching a cold in the process.
Confused by the surreal event, the protagonist confides in his skeptical but pragmatic friend, Kamiya Yutaro, at the university cafeteria. Yutaro, though disbelieving, offers practical advice. Driven by a sense of unease and responsibility, the protagonist attempts to research unusual deaths online but finds no relevant information, leading him to suspect information control. He decides to take matters into his own hands, purchasing a ghillie suit, night vision scope, and Venetian mask with his part-time job earnings to secretly monitor the river where the incident occurred. He believes he cannot simply let Satsuki bear the burden alone, sensing a hidden "trap" in the situation.
After a month of uneventful surveillance, the protagonist witnesses another monster, a pig-faced Orc wielding a trident, emerge from the river. Satsuki reappears, swiftly dispatching the Orc with her fire magic, expressing dissatisfaction with the low "experience points" it offered and lamenting a past incident where a witness (implied to be the protagonist) escaped. After Satsuki departs, the protagonist, feeling pity for the still-suffering Orc, finishes it off with its own trident. This act triggers a shocking discovery: a "Level Up" message appears in his vision, revealing he gained a level, increased stats (Strength, Speed, Luck), acquired a "Personal Status Display" skill, and changed his class from "Novice" to "Assassin," gaining the "Hidden Weapon" skill (an item box for weapons). He shares this with Yutaro, who, despite his skepticism, suggests measuring his grip strength for future comparison.
The protagonist continues his surveillance, now with a new understanding of the world's hidden mechanics. He soon witnesses a massive monster horde, including dozens of humanoids and giant anacondas, emerge from the river. This time, Satsuki is joined by another "magical girl," Ajisai, an ice mage in a blue kimono. The two girls, despite their competitive bickering over "kills" (experience points), efficiently decimate the monsters. The protagonist realizes that magical girls hunt for personal gain (leveling up) but inadvertently protect the city.
However, the true horror unfolds after the magical girls leave. The protagonist overhears a conversation between three powerful, non-human entities: a calm "Master," an aggressive "Gilku," and a subservient "Gekkei." He discovers that the magical girls are not heroes but "livestock" being "cultivated" by these "otherworlders" for their experience points. Gekkei is responsible for selecting "inconsiderate schoolgirls" and preventing information leaks. The entities plan to repeatedly kill and exploit the magical girls, even mentioning sexual assault, to gain levels, boasting a "cursed tool" that nullifies magic from level 70 and below, rendering the magical girls defenseless. The protagonist, horrified and paralyzed by fear, attempts to assassinate the Master with the Orc's trident using his 'Hidden Weapon' skill, but he freezes, unable to move. As dawn breaks, the entities depart, leaving the protagonist in despair, realizing his powerlessness and the grim truth behind the magical girl system.
