The 777 Express has no specific destination or arrival time. It goes where its passengers' fates takes it. As Monabu heads back to his duties with the SDF, he finds himself on this train! In the far future Mankind has spread out across the stars, with his main means of interstellar transport being the Galaxy Railways. Pirates and bandits prey on this interstellar commerce, which mandates the existence of two forces designed to combat such threats: the SDF (Space Defense Force), which is charged with protecting civilians and recovering lost or damaged trains, and the SPG (Space Panzer Grenadiers), an elite offensive force charged with taking the offensive against lawless elements.