It is the year 80 of the universal century, the first day of which was the end of the One Year War.

Humanity had moved the majority of its population into enormous colony cylinders in space at the LaGrange points of Sol before the start of the one year war. It was the first war in space and the first use of giant man-shaped machines of war, called Mobile Suits. The war began when the colony of Side 3, known as the Principality of Zeon, declared war on the Earth Federation with the stated cause of declaring freedom for Spacenoids from the rule of the Earth Federation. But the leadership of Zeon cared only for its own people and used deadly gas attacks on colony cylinders to empty them out for use by their own population, or outright destroyed colonies, killing billions. The Earth Federation retaliated with its own weapons of mass destruction in space, causing immense collateral casualties. The final use of WMDs had been the Principality of Zeon dropping an entire colony cylinder to earth from space, destroying the city of Sydney and permanently changing the climate for much of Asia and completely ruining the habitability of Australia. Half of all humanity died before a treaty was signed banning the use of WMDs and chemical weapons, and this was all in the first few months of the one year war.

But now the one year war that resulted in the extinction of fully half of humanity is officially over.  The Principality of Zeon falls with the death of its autocratic leaders, the Zabi family, at the hands of the ace Zeon pilot Char Aznable "the Red Comet", and so the Republic of Zeon takes its place. The Earth Federation and the Republic of Zeon sign an armistice and the war officially ends. But, not all the fighting has ended quite yet...

The federation war hero Amuro Ray, pilot of the legendary mobile suit the Gundam, and his fellow crewmates from the now destroyed space carrier ship the White Base cannot relax just yet. While the leadership of the Principality of Zeon is dead, many pockets of Zeon soldiers fight on in the name of the dead Zabi family. The crew of the White Base are assigned a new and improved ship, The White Base Kai, given new crew to replace their losses during the war, and the Mobile Suit Gundam is replaced, before being sent on a mission of crushing leftover pockets of Zeon resistance. Months pass of relatively uneventful combat, the new White Base Kai and the replacement Gundam more than enough to destroy the odd guerilla fighter piloting outdated Zeonic Zaku II mobile suits, when the Earth Federation gives them a new mission...