"Sound of the Sky" is set in a post-apocalyptic world where humanity has been ravaged by war, leaving behind a fractured society. The story centers on Kanata Sorami, a young girl who joins the 1121st Platoon of the Helvetian Army, stationed in the secluded town of Seize. Inspired by a mysterious trumpeter she encountered as a child, Kanata dreams of learning to play the bugle and joins the platoon to fulfill this aspiration. The platoon, known as the "Clocktower Fortress Platoon," is a small, all-female unit tasked with maintaining peace and order in the town, though their duties are often mundane.

The platoon consists of Rio Kazumiya, the strict and disciplined sergeant; Felicia Heideman, the kind and nurturing captain; Noel Kannagi, a quiet and skilled mechanic; and Kureha Suminoya, a cheerful and energetic soldier. Together, they form a close-knit group, balancing their military responsibilities with everyday life in Seize. The town itself is a peaceful, almost idyllic place, contrasting sharply with the war-torn world beyond its borders.

As the series progresses, the platoon encounters various challenges that hint at the larger conflicts affecting the world. They discover remnants of advanced technology from a bygone era, including a mysterious tank-like vehicle called the "Tankette," which Noel repairs and maintains. The platoon also interacts with neighboring factions, such as the Roman Empire-inspired Helvetian Army and the technologically advanced Bezel Empire, whose tensions occasionally spill into Seize.

A significant subplot involves the discovery of a young girl named Yumina, who possesses a unique connection to the world's history and the origins of the war. Her presence raises questions about the nature of humanity's past and the potential for reconciliation in a divided world. The series explores themes of hope, resilience, and the importance of human connection, as the characters navigate their roles in a fragile society.

The narrative is punctuated by moments of introspection and character development, particularly for Kanata, who grows from a naive recruit into a more confident and capable member of the platoon. The bond between the characters deepens as they face both external threats and internal struggles, ultimately highlighting the strength of their unity.

"Sound of the Sky" blends slice-of-life elements with a broader exploration of its post-apocalyptic setting, creating a story that is both intimate and expansive. The series balances its quieter, character-driven moments with occasional bursts of action and intrigue, offering a nuanced portrayal of life in a world recovering from devastation.