"Double Fake: Under the Gundam" is a manga set in the Universal Century timeline of the Gundam universe, taking place after the events of "Mobile Suit Gundam ZZ" and before "Mobile Suit Gundam: Char's Counterattack." The story centers on the Earth Federation's efforts to maintain peace and stability in the aftermath of the Gryps Conflict and the First Neo Zeon War. The narrative follows a group of characters involved in the development and deployment of advanced mobile suits, particularly the prototype Gundam Double Zeta (ZZ) and its successors.

The protagonist, Lane Aime, is a young engineer and pilot working for the Earth Federation. Lane is tasked with testing and refining the new Gundam prototypes, including the Double Fake, a mass-produced version of the ZZ Gundam. The story explores Lane's growth as a pilot and engineer, as well as his interactions with other key characters, such as the Federation officer Rezin Schnyder and the enigmatic pilot Chara Soon. These relationships are tested as the group faces threats from remnants of Neo Zeon and other factions seeking to exploit the chaos for their own gain.

The plot delves into the political and military tensions of the Universal Century, highlighting the Earth Federation's struggle to maintain control while dealing with internal corruption and external threats. The manga also examines the ethical implications of developing advanced weaponry, as Lane grapples with the consequences of his work. The action sequences feature intense mobile suit battles, showcasing the capabilities of the Double Fake and other experimental units.

Throughout the series, themes of loyalty, sacrifice, and the cost of war are explored, with the characters often forced to make difficult decisions in the face of overwhelming odds. The story builds toward a climactic confrontation that tests the limits of the Double Fake and its pilots, while also setting the stage for future conflicts in the Gundam timeline. The manga provides a detailed look at the technological advancements and political machinations of the Universal Century, offering a compelling addition to the Gundam franchise.