In the future, the planets are connected with a massive galactic railway system. The passengers of the railways are protected by two forces, the Space Panzer Grenadiers (SPG) and the Space Defense Force (SDF). This is the story of Manabu Yuuki, a new member of the Sirius Platoon within the SDF.

He and his new crewmates meld together as a team through their dealings with escaped criminals, meteors, hostile aliens, a time vortex, ghosts and terrorists before defending mankind from its greatest peril yet.