"Mobile Suit Gundam Thunderbolt: December Sky" is set during the One Year War, a pivotal conflict in the Universal Century timeline between the Earth Federation and the Principality of Zeon. The story unfolds in the Thunderbolt Sector, a hazardous region of space filled with debris from destroyed colonies and warships, making it a strategic battleground. The Federation's Moore Brotherhood, a specialized unit, clashes with Zeon's Living Dead Division, a group of amputee soldiers equipped with advanced prosthetics and mobile suits.

The protagonist, Io Fleming, is a skilled but reckless pilot from the Moore Brotherhood. He pilots the FA-78 Full Armor Gundam, a heavily armored mobile suit designed for high-risk combat. Io's brash personality and obsession with jazz music contrast with his tactical prowess, making him a formidable yet unpredictable force. On the Zeon side, Daryl Lorenz, a sniper piloting the MS-06R Zaku II High Mobility Type, serves as the primary antagonist. Daryl is a composed and disciplined soldier who lost his limbs in battle, relying on advanced prosthetics to continue fighting. His calm demeanor and sharpshooting skills make him a stark counterpart to Io.

The narrative explores the brutal realities of war, focusing on the psychological and physical toll it takes on soldiers. Both Io and Daryl are driven by personal motivations—Io seeks to prove his superiority and survive the war, while Daryl fights to protect his comrades and reclaim his humanity. Their rivalry intensifies as they repeatedly face off in the Thunderbolt Sector, each encounter escalating in intensity and stakes. The story also delves into the broader conflict, highlighting the desperation and sacrifices of both sides as they vie for control of the sector.

Themes of loss, resilience, and the dehumanizing effects of war are central to the narrative. The Thunderbolt Sector itself serves as a metaphor for the chaos and destruction of war, with its hazardous environment mirroring the internal struggles of the characters. The series balances intense mobile suit battles with quieter, introspective moments, providing a nuanced portrayal of the human cost of conflict. The animation and sound design enhance the tension, with jazz music playing a significant role in setting the tone and reflecting Io's personality.

"Mobile Suit Gundam Thunderbolt: December Sky" is a self-contained story that expands the Universal Century lore while maintaining a focus on its central characters and their personal journeys. It offers a compelling exploration of war's impact on individuals, framed within the larger context of the One Year War.