An ordinary high school student, Yuji Kaito, finds himself caught in a mass summoning event that pulls an entire classroom into a fantasy world. Unlike his classmates who are granted specific heroic classes and clear roles, Yuji receives no initial explanation or status screen. He quickly discovers, however, that he has been given a seemingly minor skill: the ability to perfectly understand and negate any magical or cursed effect applied to himself. This ability proves to be a fundamental cheat, as it makes him immune to all forms of magic, including status ailments, elemental attacks, and even divine judgment. The kingdom that summoned them expects the students to act as champions against a growing demon threat, but the royalty quickly dismisses Yuji as a useless failure due to his unimpressive starting stats. Left to fend for himself, Yuji leaves the capital and begins a solitary journey.

As he travels, Yuji realizes his negation power also allows him to absorb and repurpose the magical energy he nullifies, gradually boosting his own physical and magical parameters far beyond normal limits. His first major arc involves saving a half-elf girl named Lilia from slavers. Lilia is a former priestess of the water spirit, cursed and exiled for a crime she did not commit. Yuji nullifies her curse, and she becomes his first ally, acting as a guide to the continent's politics and hidden dungeons. Together, they investigate the source of the twisted mana that is corrupting monsters and turning them aggressive. Their exploration leads them to an abandoned dwarven forge-city, where Yuji awakens an ancient golem that recognizes him as a "void master," a rare being that can reset magical anomalies.

The second major narrative arc shifts focus to the political scheming of the summoning kingdom. The king and his court mage, who orchestrated the summoning, are revealed to be draining the life force of the other summoned students to fuel a ritual that would open a permanent gate to Earth for conquest. Yuji, now noticeably powerful, returns to the capital not for revenge but to dismantle the ritual. His confrontation with the court mage exposes that his cheat ability was not an accident; it was a failsafe encoded in the summoning circle by a previous summon victim, meant to create an individual who could shut the system down. Yuji manages to sever the mana link, freeing his classmates but also destabilizing the dimensional barrier, which attracts the attention of an elder dragon that serves as the world's guardian.

In the third arc, the elder dragon tests Yuji not through combat but through a series of mental labyrinths that force him to confront his own indifference toward the world. This challenge reveals that Yuji's emotional detachment is his true weakness, as his power grows proportionally to his will to act. He passes the trial by choosing to protect not just Lilia or his classmates, but the very land that has been ravaged by the kingdom's corruption. The dragon grants him a fragment of its core, which merges with his negation ability and gives him the new power of "void creation"—the ability to temporarily erase and rewrite magical laws in a small area, effectively making him a reality-warper. However, using this power shortens his lifespan, creating a personal stakes system that prevents him from relying on it carelessly.

The final major arc deals with the aftermath of the kingdom's collapse, as a neighboring empire invades under the banner of holy justice, secretly aiming to seize the dragon's core fragment. Yuji and his group, now including several reformed classmates and the liberated golem, wage a defensive war that forces him to use his void creation multiple times. Each use eats away at his life force, driving Lilia to seek a forbidden purification ritual from the water spirit she once served. The climax sees Yuji confronting the empire's supreme general, who wields an anti-magic armor identical to Yuji's original negation. This leads to the revelation that the general is another summoned individual from centuries past, who also had the negation skill but used it to become a tyrant. Yuji, facing a distorted mirror of himself, chooses to erase the general's magic absorption ability rather than destroy him, depowering the threat permanently. The series closes with Yuji restoring the land's mana balance, but choosing not to return to Earth, instead staying in the fantasy world to dismantle the lingering summoning circles that could pull in more unsuspecting people. He and Lilia establish a neutral territory for displaced summons, and his cheat remains a quiet secret known only to his closest allies, as he decides that ultimate power is best used sparingly.