Guy Gullveig was bullied by his older half-brother for being an illegitimate child. Guy was taken in by his grandfather, a former adventurer living deep in the mountains, and began training with "practice swings" as part of his mental and physical healing. Even after his grandfather passed away, Guy continued his routine of 10,000 practice swings a day until he became an "old man" (Ossan). One day, Guy heads to the city aiming to become an adventurer, as his grandfather had once encouraged. But Guy was the only one who didn't realize—thanks to the practice swings he kept doing, he had become the strongest swordsman! The story of an "old man" adventurer who put in too much effort with 10,000 swings a day, defeating powerful foes with his well-trained sword—an unconscious "Musou" Fantasy!!