Over three centuries have elapsed since the Calamity War, a devastating conflict between Earth and its extraterrestrial colonies. Currently, Earth is governed by four economic blocs, while Gjallarhorn serves as the military organization responsible for maintaining peace. In contrast, Mars relies heavily on Earth's economy. Kudelia Aina Bernstein, an aristocrat from Chryse Autonomous Region, becomes involved in Mars' independence movement due to her horror at the deplorable living conditions of its inhabitants. To achieve this goal, she hires Chryse Guard Security (CGS) for protection during her journey to Earth to negotiate economic terms with the controlling bloc. The Third Army Division, comprising Mikazuki Augus, Orga Itsuka, and other child soldiers, is assigned to safeguard her.
When Gjallarhorn launches an attack on CGS facilities in an attempt to assassinate Kudelia, Orga and his companions must prevent the attackers from succeeding. Their actions might unintentionally catalyze a turning point that leads the children to forge their own destiny.