As a plot to kill both of them, Reinhard von Müsel and Siegfried Kircheis are dispatched by their superior to reconnoitre deep into the planet Kapche-Lanka for Free Planets Alliance activity all by themselves. Before their departure, Reinhard and Kircheis' tank is tampered with to ensure that they will not have enough fuel to make it back to base. It is nightfall before both of them realise that act and they are forced to halt and figure a way to get back to base alive. By chance, they encounter an Alliance tank advance unit and decides to ambush it for securing fuel.
Prequel to the original series, focusing on Reinhard and his steps up the chain of command in the Galactic Empire. Also focusing on the FPA's Yang Wen-li. A chronicle on how he earned the title "The Hero of El Facile" and his handling of assignments he received after the historic event. Spiral Labyrinth also continues with the background of Reinhard and Kircheis where the first Gaiden left off.
