"Sound of the Sky" is set in a post-apocalyptic world where humanity struggles to rebuild after a devastating war. The story centers on Kanata Sorami, a young girl who joins the 1121st Platoon of the Helvetian Army in the secluded town of Seize. Inspired by a trumpeter she encountered as a child, Kanata dreams of learning to play the bugle. The platoon, stationed in Seize, is tasked with maintaining peace and order, though their duties are often mundane, such as patrolling the quiet town and assisting its residents.

The 1121st Platoon consists of a small group of soldiers: Rio Kazumiya, the strict and disciplined sergeant; Kureha Suminoya, a cheerful and energetic private; Noel Kannagi, a reserved and skilled mechanic; and Filicia Heideman, the kind and nurturing captain. Together, they form a close-knit unit, balancing their military responsibilities with their personal lives. The narrative explores their daily routines, interactions with the townspeople, and the bonds they share as they navigate the challenges of their world.

As the story progresses, subtle hints about the larger world emerge. The war that ravaged the planet left behind remnants of advanced technology, including autonomous war machines known as "Tanks." These machines, now dormant, serve as a reminder of humanity's past conflicts. The platoon occasionally encounters these relics, which add an undercurrent of tension to their otherwise peaceful existence. The series also delves into the cultural and historical aspects of the world, particularly through the "Amber Whisper," a mysterious melody said to have the power to bring peace.

Kanata's journey to master the bugle becomes a central theme, symbolizing hope and resilience. Her determination to learn the instrument reflects her desire to connect with others and bring comfort in a fractured world. The narrative intertwines moments of lighthearted camaraderie with deeper reflections on loss, memory, and the enduring human spirit. Through its exploration of both personal and collective struggles, "Sound of the Sky" presents a poignant tale of perseverance and the enduring power of music in a world striving to heal.