The year is Universal Century 0111. Decades have passed since the conflicts of the First and Second Neo Zeon Wars, and the Earth Federation has grown complacent. The primary mission of the Federation's test corps is no longer frontline combat but the rigorous evaluation of next-generation mobile suits for mass production. Among these prototypes is the F90, a highly modular unit designed for rapid mission-specific refits. However, this peacetime routine is shattered when a rogue faction known as the Oldsmobile Army, composed of zealous remnants from the Principality of Zeon's aristocratic families, launches a surprise assault on the Federation's lunar testing facility. Their goal is not conquest but the seizure of the F90 and its critical combat data, which they intend to use as the catalyst for a new war to reclaim spacenoid independence.

The narrative follows Ensign Def Stallion, a young and idealistic Federation test pilot who finds himself thrust into a desperate struggle after the Oldsmobile attack. As the facility falls, Def is forced to sortie in the partially damaged F90 to protect his comrades and prevent the suit from falling into enemy hands. He is joined by his colleague, Ensign Sisi Quinton, a sharp and pragmatic pilot who operates a support unit. Together, they flee the lunar base, becoming fugitives pursued by the relentless Oldsmobile forces. Their journey takes them across the Earth Sphere, from the debris-strewn shoal zones to the colonies of Side 4, as they attempt to reach Federation headquarters with the F90's data.

The central conflict escalates as the Oldsmobile Army deploys their own powerful mobile suits, including the advanced RF (Refined) series, which are upgraded versions of classic Zeon designs. The faction is led by the charismatic but ruthless Commander Metz Loher, who believes that only through conflict can the Spacenoid spirit be reignited. Opposing him from within the Oldsmobile ranks is Sergeant Major Bromb Texter, a veteran pilot who grows increasingly disillusioned with the faction's extremist methods and the senseless loss of life. As Def and Sisi fight for survival, they are drawn into a complex web of loyalties, with Bromb eventually questioning his own allegiances and becoming an unlikely, temporary ally.

The manga is structured around several key mission arcs, each defined by a different configuration of the F90's interchangeable packs—such as the high-mobility A-type or the heavy-assault V-type—forcing Def to adapt his tactics to diverse combat environments, including a harrowing battle inside a colony cylinder and a desperate fight on the surface of Mars. The narrative delves into the psychological toll of war, contrasting Def's naive sense of duty with Sisi's colder, survival-focused realism. It also explores the Oldsmobile Army's fractured ideology, revealing that many of its soldiers are not monsters but misguided patriots clinging to a romanticized past. A notable subplot involves the pursuit of the F90 by a Federation special forces unit, which views Def and Sisi as deserters, adding a layer of paranoid thriller to the war drama.

As the conflict reaches its climax, Def is forced to confront the true nature of the F90's secret data, which contains not just performance metrics but a devastating tactical system that could tip the balance of power forever. Rather than deliver this weapon to either the corrupt Federation or the vengeful Oldsmobile forces, Def makes a critical choice to destroy the core data, accepting the consequences of insubordination to prevent another catastrophic war. The final arc sees a three-way confrontation at the Oldsmobile's asteroid base, where Def, Sisi, and a now-revolted Bromb must work together to stop Metz Loher's final, suicidal gambit. The manga concludes not with a grand victory, but with a fragile ceasefire, leaving Def and Sisi as disillusioned veterans who have preserved peace at the cost of their careers and innocence, setting the stage for the larger political upheavals that will define the coming decade.