The world of Guilty Gear is an earth-like world. The use of circuit-based technology has been outlawed, in favor of the endless resources of Magic taught to the world by several Sages over a century ago. Magical-technology has allowed 'modern' technology to flourish, with airships, TV, radio, and other modern conveniences running on the power of magic stemming from the parallel universe, The Backyard.
But though the differences between people have lessened thanks to this Magic, war continues. Man used this power to create biological weapons known as Gears, that themselves rebelled against Man's slavery. This launched the conflict known as the Crusades, where Gears and Humans alike fought to the point of nearly destroying the world.
However, the former Holy Knight and original researcher on the Gear Project fought and killed his lover and Commander Gear, Aria, known as Justice. This largely ended the Crusades, and in the chaos that followed, the United Nations Senate and the Post-War Administration Bureau manipulated by the group called the Conclave began to covertly take over the world.
With the world in hand, and the emergence of the creatures known as Valentines, the Conclave finally moves forward with their goals: to take over the world fully, and direct the evolution of humanity by removing free-will. To do this, they require a soul to inhabit the body of the deceased Justice, and the pure energy that exists within the Backyard.
Even as the Conclave works against the world, the Valentine Ramlethal declares war on the world as she allied with the Conclave. The only ones with enough information to stop the Conclave are the King of Illyria, Ky Kiske, and his allies, and Elphelt Valentine who works to save and stop her sister from destroying humanity.