"Mobile Suit Gundam Side Story: Hidden Shadow G" is a manga series set in the Universal Century timeline of the Gundam franchise. The story takes place during the One Year War, a conflict between the Earth Federation and the Principality of Zeon. The narrative centers on the Earth Federation's secret development of advanced mobile suits and the covert operations surrounding them.

The protagonist, Shigeru Akiba, is a skilled test pilot assigned to the Federation's hidden research facility, codenamed "Shadow G." The facility is tasked with developing the RX-78GP00 "Blossom," a prototype mobile suit designed to surpass the capabilities of the RX-78-2 Gundam. Shigeru's role is to test and refine the Blossom, ensuring its readiness for deployment. However, the project is shrouded in secrecy, and Shigeru becomes increasingly aware of the moral and ethical dilemmas surrounding the development of such advanced weaponry.

As the war intensifies, Zeon forces discover the existence of the Shadow G facility and launch an assault to capture or destroy the Blossom. Shigeru is thrust into combat, piloting the Blossom to defend the facility and its personnel. The story explores the psychological toll of war on Shigeru, as well as the broader implications of technological advancement in warfare. The Blossom's capabilities are both a blessing and a curse, as its power comes with significant risks and consequences.

Supporting characters include Dr. Yuki Minagawa, the lead scientist overseeing the Blossom project, and Lieutenant Commander Ryu Jose, a seasoned Federation officer who provides guidance and mentorship to Shigeru. The antagonists are primarily Zeon pilots and commanders, who are determined to secure the Blossom for their own purposes. The narrative delves into the motivations and struggles of both sides, highlighting the human cost of the conflict.

The manga balances action-packed mobile suit battles with introspective moments, examining themes of duty, sacrifice, and the impact of war on individuals. The Blossom's development and deployment serve as a focal point for the story, driving the plot forward while raising questions about the ethics of warfare and technological innovation. The series maintains a grounded approach to the Gundam universe, focusing on the personal experiences of its characters within the larger context of the One Year War.