"Gunparade Orchestra" is set in an alternate history where Earth is under constant threat from extraterrestrial beings known as Phantom Beasts. These creatures emerged in 1945, disrupting human civilization and forcing humanity to develop advanced mecha units called Humanoid Walking Tanks (HWTs) to combat the invasion. The story takes place in 1999, focusing on a group of young soldiers who are part of the Humanoid Walking Tank Corps, tasked with defending Japan from the Phantom Beasts.

The narrative centers on the 108th Training Unit, a group of high school students who are drafted into military service due to the severe shortage of adult soldiers. The unit is stationed at Fuji Gekkou Academy, where they balance their military duties with their education. The protagonist, Atsushi Hayami, is a reserved and introspective young man who struggles with the weight of his responsibilities as a soldier. Alongside him are his fellow cadets, including the determined and skilled Mai Shibamura, the cheerful and optimistic Yayoi Kunugi, and the stoic and disciplined Shingo Kuwabara. Each character grapples with the emotional and psychological toll of war, as well as the challenges of adolescence.

The plot unfolds as the 108th Training Unit is deployed on various missions to repel Phantom Beast attacks. These missions highlight the camaraderie and tensions within the group, as well as their growth as soldiers and individuals. The series explores themes of sacrifice, duty, and the impact of war on young lives, emphasizing the moral complexities of their situation. The Phantom Beasts, while initially portrayed as mindless invaders, are later revealed to possess a deeper connection to Earth, raising questions about humanity's role in the conflict.

As the story progresses, the unit faces increasingly dangerous threats, culminating in a climactic battle that tests their resolve and unity. The series balances action-packed mecha combat with introspective character moments, offering a nuanced portrayal of its young cast. "Gunparade Orchestra" stands out for its blend of alternate history, mecha action, and character-driven drama, providing a thoughtful exploration of its central themes.