Long ago, humanity angered the gods and lost their ability to use magic. Refusing to accept this, humans discovered a way to seal magic into bullets, which could then be fired from guns to cast spells. This led to a new era where magical power is determined by the bullets one carries, plunging the world into constant warfare.

In this world, a young魔枪手 (magic gunner) named Cedric lives with his older sister Elwing, a nun, in the Mukanarina Holy Kingdom. Cedric believes his family was killed by bandits and has been raised in a monastery. However, the infamous criminal魔枪手 known as Olivier, also called Olivier the Chimera, steals a legendary magical gun known as the Gun Princess from the Holy Kingdom's headquarters. The Gun Princess is an ancient artifact with the terrifying power to erase any single word from existence, thereby causing whatever that word represents to vanish from the world, be it a person, a nation, or even a concept.

Cedric, along with his sister Elwing and a skilled young female魔枪手 named Ambrosia, is tasked with pursuing Olivier and recovering the Gun Princess. As they journey together, Cedric learns from Olivier that his family was not killed by bandits but was exterminated by the Mukanarina Holy Kingdom itself. This revelation causes Cedric to lose control of his magical power, unleashing a devastating outburst. Ambrosia, whose full name is Ambrosia de Michel Galliard Edna Elisa Ambrosia, harbors her own secret; she is a princess of the fallen fortress nation of Galliard and seeks the Gun Princess to take revenge on the Srafat Empire that destroyed her homeland and to restore her kingdom. She bears a scar on her chest from a magical burn inflicted by the Dragon King of Srafat.

Their journey unfolds against the backdrop of a world where magic is accessed through firearms.魔枪手 must perform a ritual called sealing to enchant silver bullets with spells, chanting ancient words to capture magical elements from their surroundings. The effectiveness of a spell depends on the purity of the gunner's magical lineage, leading to the existence of bloodline refinement facilities that breed humans for powerful magical traits. The party travels through various nations, including the Dawn Empire and the Moon Sea Kingdom, encountering allies such as the fire mage Barbalius Neo, known as the Skull King, and enemies like Keith Basilis, the Red Mane, an officer of the Srafat military who loses his high rank after being defeated by a magically awakened Cedric in a duel.

Throughout the series, Cedric, Ambrosia, and Elwing pursue Olivier while confronting the dark secrets of their own pasts and the political machinations of the nations that seek to control the Gun Princess for themselves. The journey forces them to question who truly deserves to wield the power to erase anything from reality, as they travel across a war-torn fantasy world where every gunshot carries a spell that can change the course of history.