"Gunparade March" is set in an alternate history where humanity faces an ongoing war against mysterious alien creatures known as Phantom Beasts. The story unfolds in 1999, decades after the Phantom Beasts first appeared, decimating much of the world's population and forcing humanity into a prolonged struggle for survival. To combat this threat, Japan establishes the Humanoid Walking Tank (HWT) units, piloted by young soldiers who are often teenagers conscripted into service. These pilots operate advanced mechs called Unit 12s, designed to fight the Phantom Beasts.

The narrative centers on the 5121st Platoon, a group of young soldiers stationed in Fuji, Japan. The platoon is composed of diverse characters, each with their own motivations and struggles. The protagonist, Atsushi Hayami, is a reserved and introspective young man who struggles with the weight of his responsibilities as a soldier. His interactions with his fellow platoon members, including the spirited and determined Mai Shibamura, form the emotional core of the series. Mai, who initially clashes with Atsushi due to their differing personalities, gradually develops a deeper connection with him as they face the horrors of war together.

The series balances its wartime narrative with slice-of-life elements, exploring the daily lives of the platoon members as they navigate the challenges of adolescence while fulfilling their military duties. Themes of camaraderie, loss, and the psychological toll of war are prominent, as the characters grapple with the realities of their situation. The Phantom Beasts remain a constant threat, and the platoon is frequently deployed on missions to protect their homeland, leading to intense battles that test their resolve and teamwork.

As the story progresses, the platoon faces internal and external conflicts, including tensions within their ranks, the pressure of their superiors, and the ever-present danger of the Phantom Beasts. The series also delves into the broader implications of the war, touching on the societal impact of relying on young soldiers and the moral ambiguities of their mission. Through its blend of mecha action, character-driven drama, and exploration of wartime themes, "Gunparade March" offers a nuanced portrayal of its characters and their struggles in a world defined by conflict.