The desert world is a lawless expanse ruled by desperadoes, outlaws, and brigands for centuries. The legendary criminal M. Godspeed has been the most feared outlaw of all, but he is killed by his own son, Jimmu Godspeed. Jimmu drags his father's coffin through the harsh desert, claiming the ten-million-dollar bounty on his head not for money but to prove himself worthy of enrolling in the Western U.S. Marshal Training Institute. His true objective is to collect the scattered body parts and weapons of the first Marshal King—including the mythical super magnum known as Excalibur and the golden arms once used by his father—so he can revive the King's power and bring law and order to the age of desperadoes.

At the Institute, Jimmu's notorious heritage makes him a target among fellow students. He is taken into Class 1-C by Agera Dutch Sheaffer, a quiet observer, and immediately clashes with the skilled and proud Mira Abigail Black. When the Wrangler Gang attacks the school, Jimmu secretly retrieves Excalibur and the golden arms, defeats the gang leader, and declares his intention to become a marshal. The faculty, led by the cyborg principal Fair Lady, subject him to a series of grueling challenges: a no-guns flag recovery, a thirteen‑gunfighter fast‑draw, a team battle, and an endurance duel against Mira that ends when Jimmu refuses to strike her. He earns his Marshal badge and forms the Adventure of Godspeed Posse Prep Club with Agera and Mira.

Their first mission as a team takes them to Devil's Canyon, where they confront a masked monster said to have devoured hundreds. Jimmu discovers that the creature is actually Nano Serizawa, a man who awoke from a long slumber a century ago and was forced into hiding after hostile miners attacked him. Nano pleads to be left alone, but Jimmu's pursuit of relics and companions continues as the group sets out to gather the remaining pieces of the Marshal King, facing ever‑greater dangers in a world where steampunk technology, gunfights, and superhuman abilities collide.