Approximately during the early years of 2500, humanity established a number of colonies within another solar system. Technological advancements and cybernetics were abundant, though as were new foes for these Terrans. With the emergence of a new species, the Terrans were in disarray, especially after the seemingly unwarranted culling of an entire Terran colony. After sending a clumsy counter attack, this new species, calling themselves the Protoss, mysteriously withdrew from the orbit of a second colony about to meet the same fate. However, shortly after their departure a new insect-like species, one that would later be named the 'Zerg', revealed themselves and destroyed all life upon the colony.
With the unexpected coming of these two new alien species, a war for supremacy soon broke out. However, after time the war swiftly changed to one of survival. While the species known as the Protoss sought after the preservation of life, the carnivorous Zerg sought to fulfill their destiny as the 'perfect life-form' by assimilating and destroying all other life in existence. During the battles, the Confederacy was slowly taken over by the Sons of Korhal, forming the "Terran Dominion", and the Zerg pushed back, though many losses and sacrifices were made. One of these sacrifices was key to the Zerg Overmind and his 'Cerebrates', the Hive Minds of the Zerg Swarms. This sacrifice, a ghost by the name of Sarah Kerrigan, was easily overtaken and began her transformation to suit the Overmind's new plans.
Upon the entrance of the banished brethren of the Protoss, the location of their home world, Aiur, was finally revealed to the Zerg, seeing them as key to their Ascension to the perfect life-form. This information was leaked unwilling via one of the 'Dark Templar', unjustly banished long before the Zerg threat. With this new knowledge, the Zerg invaded Aiur, easily overwhelming all in their path. It was only after the Protoss accepted the aid of the Dark Templar, that they were able to free their world from the Overmind. With the loss of their central hive mind, the varying Zerg swarms fell into chaos and killed all they met without purpose, while Sarah Kerrigan began her Ascension to the Queen of Blades.
With the Protoss world of Aiur desecrated by the Zerg Swarms, the Protoss and their Terran allies leave the planet to move to the Dark Templar home world, Shakuras. Once they reach the planet, however, they realize the Zerg had used the very method of their escape to follow them. Making her appearance to the Protoss, Kerrigan aids them in gathering up the necessary crystals to protect their planet, though only with a secret agenda as well as being unable to control the ravenous swarms. Once the crystals were retrieved, the Protoss were able to completely annihilate the Zerg presence upon their planet.
However, during the confusion, the UED (United Earth Directorate) exits out of warp nearby the Terran colonies and begins to assault the Terran Dominion. However, upon their encounters with the Zerg, they begin to experiment on them during their campaign, eventually attempting to control them, leading to the ultimate capture of the newly developing Overmind. The UED takes command of almost all the worlds of the Terran Dominion, creating a sense of unity between the colonies beneath their command, as well as establishing a presence for the Earth.
The Protoss, along the Terrans and Kerrigan herself then move to assault the Overmind. Once the Overmind is destroyed along with most of the UED fleet, Kerrigan turns on the Protoss and Terrans, slaying many heroes who have evaded death to defend their race. With the Overmind destroyed, Kerrigan gives the broken UED fleet a chance to flee, however as they flee, with a swift stroke she completely annihilate the UED and takes over planet Tarsonis as her home world. Finally controlling all the Zerg swarms, Kerrigan allows the Protoss and Terrans to lick their wounds, as dark shadow looms over the horizon.