The story centers on Dellate, a 40-year-old blacksmith who has worked for the local guild for 25 years under a personal philosophy of never trying harder than necessary, preferring to spend his free time crafting toy weapons for the neighborhood children. After a prideful colleague becomes the new guild master, Dellate is suddenly expelled from his position. He then remembers a letter he received from one of those children, now grown, and decides to travel to the royal capital.

Upon arriving, Dellate discovers that the simple toy weapons he made years ago have somehow evolved into legendary S-rank equipment, and the children who received them have all become powerful national heroes. One of these former children, a woman named Militar, has risen to become an army corps commander and recruits Dellate as the exclusive blacksmith for the national army. Soon after, he is challenged to a forging duel by Nanahikari, a blacksmith who obtained her position through family connections but loses spectacularly when Dellate demonstrates his true skill.

The narrative progresses through several arcs. After establishing his position with the military, Dellate encounters a second former child, Shinion, who has become a top instructor at the magic academy and is dealing with a malfunctioning evolved weapon. Dellate helps resolve both her equipment issues and a conspiracy threatening to overthrow the nation. He then reunites with a third former child, Beionette, now captain of the markswomen corps, who requests repairs for her malfunctioning S-rank rifle. Meanwhile, the guild master who expelled Dellate attempts to repair a national treasure sword but destroys it in seconds, losing his position and being ordered by the queen to become Dellates disciple as punishment. The story unfolds in a fantasy world featuring a blacksmith guild system, a royal capital, military headquarters, a magic academy, and an encounter with an A-rank monster, the Iron Bear, which Dellate defeats with a single blow from one of his crafted weapons.