"Gunparade March" is set in an alternate history where humanity faces an ongoing war against mysterious alien creatures known as Phantom Beasts. These beings emerged during World War II, forcing humanity to unite against a common enemy. The story takes place in 1999, decades after the initial invasion, with the war still raging. To combat the Phantom Beasts, Japan has established the Humanoid Fighting Vehicle (HFV) units, piloted by young recruits who are often high school students. These teenagers are trained to operate advanced mechs called Unit 12s, which are humanity's primary defense against the alien threat.

The narrative centers on the 5121st Platoon, a group of young HFV pilots stationed at Fuji Gakuen, a military academy. The platoon is composed of diverse characters, each with their own motivations and struggles. The protagonist, Atsushi Hayami, is a reserved and introspective pilot who often questions the purpose of the war and his role in it. His interactions with his fellow platoon members, including the determined and idealistic Shiori Tachibana and the pragmatic Mai Shibamura, drive much of the story's emotional core. The group must navigate the challenges of adolescence while bearing the immense responsibility of protecting humanity.

The plot explores the daily lives of the 5121st Platoon as they balance their military duties with personal growth. Training exercises, missions against Phantom Beasts, and interpersonal conflicts form the backbone of the story. The series delves into themes of camaraderie, sacrifice, and the psychological toll of war, particularly on young soldiers. As the platoon faces increasingly dangerous missions, the stakes rise, and the characters are forced to confront their fears and insecurities.

A significant subplot involves the mysterious origins of the Phantom Beasts and the true nature of the war. Clues and revelations gradually emerge, hinting at a deeper connection between humanity and the alien invaders. This mystery adds a layer of intrigue to the narrative, as the characters grapple with the possibility that their understanding of the conflict may be incomplete or flawed.

The manga adaptation of "Gunparade March" expands on the world-building and character development introduced in the original video game. It provides a more intimate look at the lives of the 5121st Platoon, emphasizing their humanity and resilience in the face of overwhelming adversity. The story's blend of mecha action, personal drama, and speculative fiction creates a compelling narrative that appeals to fans of both science fiction and character-driven storytelling.