The protagonist, a 17-year-old sniper known as "Smile Reaper" for his unnerving smile even in dire situations, is tasked with eliminating a hostage-taker. He successfully shoots the perpetrator in the shoulder, allowing for the hostage's rescue. His unique personality, characterized by his constant smile and a detachment from the grim realities of his profession, is highlighted. He shares a decade-long bond with his SRR81+ sniper rifle, nicknamed "Sla-san."
During a subsequent mission, the protagonist is ambushed by multiple snipers employing an unusual "light bullet" attack that can manifest from glowing patterns on surfaces. He sustains a leg injury but discovers that destroying these patterns renders the attack inert. He manages to escape the building and engage his attackers in a tense sniper duel. He demonstrates exceptional skill and intuition, anticipating and countering their moves, eventually defeating them.
Following this intense battle, the protagonist witnesses a passenger plane about to crash. He and Sla-san leap from the rooftop to escape, and he is seemingly killed in the ensuing explosion. However, he awakens in a vast grassland, discovering that he has been transported to another world. Sla-san is gone, and he is accompanied by a young girl who claims to be the materialized form of his beloved rifle. This new world operates on RPG-like mechanics, with stats and levels.
He learns that Sla-san, now a human-like entity, can be equipped as his weapon, drawing MP to generate ammunition. They encounter a group of goblins attacking a girl named Salastia, whom they rescue. Salastia reveals the world's currency system and offers to help the protagonist, who has lost his memory of his name and past, by suggesting he visit a guild.
At the guild, the protagonist, now referred to as "Kuro," and Sla-san are identified as a "Milly-sal weapon" user, a rare phenomenon. They meet the Guild Master, Surt, a powerful elder. Kuro undergoes a combat test against a muscular opponent named Afro, whom he swiftly defeats using his sniper skills and Sla-san's abilities. He is ranked C.
Kuro receives a guild card and chooses the "Archer" role, which enhances Sla-san's abilities, granting it new skills like "Healing" and "Original Bullet." He accepts a quest to hunt Blue Tigers in a forest. He and Sla-san efficiently eliminate the tigers, but then encounter a King Goblin. Kuro, despite his low level, engages the King Goblin, showcasing his combat prowess and Sla-san's adaptability. He is severely injured but survives thanks to Sla-san's intervention and his own resilience.
Upon returning to the guild, Kuro's victory over a King Goblin, a monster typically requiring high-ranked adventurers, causes a stir. He learns that King Goblins are appearing in shallower areas, indicating a potential crisis. He is summoned to the royal castle and, using his ability to discern truth, requests the King's daughter's "eyes" (implied to be a magical ability to see the future) as a reward. He then proposes joining a temporary knight order, leveraging his teleportation ability to ensure the King's safety, which is accepted.
Kuro begins his second school life at the national magic academy. He and his companions, including his newly acquired companions Kill (a survival knife) and Nine (a handgun), continue to grow stronger. They face increasingly dangerous monsters and participate in large-scale battles, including defending the kingdom from a massive monster horde. Kuro's combat style evolves, incorporating his weapons and newfound skills, often with a characteristic smile. He develops a deep bond with his weapons, particularly Sla-san and Kill, who gain sentience and express affection. The narrative progresses through various quests and battles, highlighting Kuro's growing power and his unique, often brutal, approach to combat, all while maintaining his enigmatic smile.