"Gunparade Orchestra" is set in an alternate 1945 where humanity faces an ongoing war against extraterrestrial beings known as Phantom Beasts. These creatures emerged during World War II, forcing humanity to unite and develop advanced mecha called Humanoid Walking Tanks (HWTs) to combat the threat. Decades later, the war persists, and young cadets are trained to pilot these machines as part of the Global Defense Organization.

The story primarily follows the 108th Platoon, a group of teenage cadets stationed in Japan. The platoon is tasked with defending their region from Phantom Beast incursions while balancing their military duties with the challenges of adolescence. The cadets, including the reserved and introspective Atsushi Hayami, the determined and skilled Mai Shibamura, and the cheerful yet insecure Yayoi Kunugi, must navigate their personal struggles, relationships, and the pressures of combat.

As the series progresses, the 108th Platoon faces increasingly dangerous missions, revealing the complexities of the war and the toll it takes on both soldiers and civilians. The narrative explores themes of camaraderie, sacrifice, and the psychological impact of warfare, particularly on young soldiers. The cadets' interactions and growth are central to the story, highlighting their evolving dynamics and individual arcs.

The Phantom Beasts, while a constant threat, also serve as a backdrop to the human drama, with the series delving into the origins of the conflict and the moral ambiguities of war. The cadets' experiences are juxtaposed with glimpses of civilian life, emphasizing the contrast between their youth and the harsh realities they face.

"Gunparade Orchestra" blends mecha action with character-driven storytelling, offering a grounded portrayal of its protagonists' lives amidst a larger conflict. The series maintains a focus on the emotional and psychological dimensions of its characters, providing a nuanced exploration of their struggles and resilience in the face of adversity.