With the outbreak of the One Year War, the Principality of Zeon forms the 603rd Technical Evaluation Unit, a team of soldiers, technicians, and engineering specialists in charge of assessing various prototype weapons aboard the Jotunheim, a civilian cargo ship refitted and enlisted for military use.

As the Earth Drop Operation begins, the 603rd is assigned to test the YMT-05 Hildolfr, a massive transformable mobile tank. The task falls onto Maj. Demeziere Sonnen, a respected armored warfare instructor who is now disgraced and troubled by drug addiction from his own misery, after Zeon abandoned armored vehicles in favor of mobile suits.

Upon news of a powerful new Federation mobile suit, Zeon propagandizes the EMS-10 Zudah mobile suit as superior to the Federation prototype. The 603rd is put in charge of testing three prototypes with the help of Zimmad Company test pilot Maj. Jean-Luc Duvall. However, the testing soon goes wrong after one of the Zudahs flies out of control and disintegrates during a combat simulation against a Musai warship, killing its pilot. The weapon test is then grounded indefinitely.

With Zeon losing their foothold on Earth, the 603rd is given a new project — to infiltrate a prototype weapon into Federation airspace and attack Federation warships launching from Jaburo. Ensign Werner Holbein, an arrogant naval officer who boasts a proud maritime family tradition, is assigned to the Jotunheim to pilot the mobile diver unit Ze'Gok.

The situation for Zeon is getting desperate after the Federation successfully launches Operation Cembalo - the assault on the space asteroid fortress Solomon. Upon arriving at A Baoa Qu, the Jotunheim is visited by Col. Herbert von Kuspen, an uptight high-rank commander from an elite battalion, who immediately tries to assume command of the ship but meets resistance from both Monique and Prochnow. He brings along a squad of Oggo mobile pods and their pilots he claims are the "Principality's elite soldiers" — who are actually young cadets forcefully mobilized from the homeland. To Monique's shock, the most enthusiastic of the recruits is actually her younger brother Erwin.

Just as Gihren Zabi proudly announces to his troops the decimation of the Federation fleet by the Solar Ray, the 603rd receives its latest prototype weapon — the massive Big-Rang mobile armor.

The Federation forces are routed in face of the onslaught of the Zeon Zakus during the Earth Drop Operations. When the main forces are retreating for resupply, anti-mobile suit tactical squad commander Lt. Ben Barberry is ordered by his arrogant superior, Col. Michael Colmatta, to advance and ambush the pursuing Zeon forces. Unknown to others, Barberry, who loses every subordinate in previous engagements, is frequently haunted by a spirit called Shinigami, who patrols the battlefield collecting souls of fallen warriors.

Sgt. Rayban Surat is transferred to the 1st Platoon, 301st Tank Squadron of the Federation's 44th Hybrid Regiment as a driver for Lt. Harmon Yandell, a seasoned tank ace. However, Yandell is a man with his own demon — he survived an encounter with Zeon's feared "White Ogre" Cpt. Elmer Snell at the cost of his left leg, and sees himself as a homeless dead man walking. His previous drivers have either been killed, or gone insane seeing the Death Deity haunting Yandell's back. After seeing Yandell's irrational behavior, Surat asked his commanding superior, Col. Colmatta, to relieve Yandell's command. Instead of doing so, Colmatta approved Yandell's mad plan to launch a night assault on the oncoming Zeon force, which Snell leads.

Though Operation Odessa is proceeding, the 44th Hybrid Brigade suffers setbacks after its advance is bombarded by artillery fire from Zeon's Dabude-class land battleships. They are reinforced by three RTX-440 Guntanks (advanced versions of the RX-75 Guntank seen in the original series) piloted by a trio of convicts led by Lt. Arleen Nazon, a technical officer blamed and condemned to rot in jail after her lover, Clyde Bettany, defected to Zeon with information on the RTX-440. Arleen vows revenge for destroying her future.