In the 21st century, the United Earth Government was formed following a devastating four year civil war. The period following the formation of the UEG was known as the "Great Purification". The UEG found all of the criminals, rebels, and outcasts of Earth, and began to capture and execute them. Some years later, however, scientists on Earth discovered a strange pulsating energy source on the edge of the galaxy. UEG kept this a secret from the public but decided to send the criminals to investigate it. They put them in cryogenic stasis and loaded them onto four ships. They were sent towards the edge of the galaxy, to explore for an energy source, and when they awakened they would be known to all aliens as Terrans.

Centuries before this, a peaceful but meddlesome race named the Xel'Naga decided to attempt to create the perfect species. They endowed this species with incredible psionic powers which enabled them to move and create matter with their minds along with telepathy. As this race grew in power and sophistication, the Xel'Naga named them Protoss, or first-born, and decided to live among them. When the Xel'Naga came down from their ships, the Protoss immediately worshiped them as gods. As time passed, however, the Protoss became disunited and civil war broke out. They questioned whey the Xel'Naga had come to live among them and attacked the Xel'Naga with their mental powers, killing many and causing the rest to flee. The Xel'Naga, outraged that their perfect creations would betray them in this way sought to create a new race of beings that would have one collective mind and so could be controlled by them. They traveled to a distant world in the galaxy, using an unknown species of worm and giving it the power to absorb the genes of all species it came into contact with. They called this species the Zerg.

As the Zerg grew in complexity of genetic material, incorporating the best elements of each species that it came into contact with and destroyed, they realized the ultimate species that would make them all-powerful. The Zerg attacked the Xel'Naga, incorporating their genetic material and stealing their ships. They had but one thought - to destroy the one race more powerful than their own - the Protoss. To do this they would first need to acquire the mental abilities needed to attack the Protoss and beat their psionic defenses. They decided they need to use the Terran genetic material, which although somewhat limited, did contain enough psionic genetic material to give them a chance to get past the Protoss' defenses and become supreme in the universe. At the same time, the Protoss have discovered the Terrans as well as the Zerg threat, and have mobilized an army able to create weapons and armor out of pure psychic energy to defend their world and stop the threat to their existence.