With the zerg invading Tarsonis, the Terran Confederacy was thrown into chaos and fought a losing battle to defend the planet. As happened on previous planets the zerg infested, the protoss expeditionary fleet under the command of Executor Tassadar, arrived to stop the zerg. However, instead of purifying the planet from orbit as had been done before, Tassadar sent his forces to combat the zerg directly, attempting to spare the terrans on the planet.
Arcturus Mengsk of the Sons of Korhal feared the protoss attack would buy the Confederates time to flee the planet. Wishing to ensure the Confederacy's destruction, Mengsk ordered Lieutenant Sarah Kerrigan to take a strike force and engage the protoss to protect the zerg. Kerrigan trusted Mengsk, but Captain Jim Raynor made his disagreements with Mengsk's methods known to both of them.
Kerrigan led the Sons of Korhal against the protoss and defeated their forces, and the rest of the expeditionary fleet retreated. However, the terrans were caught in a massive wave of zerg that advanced on their base. Mengsk ordered the Sons of Korhal to withdraw from the planet, abandoning Kerrigan to the zerg.
The zerg invasion continued, and the Terran Confederacy's capital was lost. The zerg would continue to rampage unchecked across Tarsonis for months. Kerrigan was presumed dead, but the zerg captured and infested her, creating the Queen of Blades.
The player's starting position is delicate. There is a zerg base to the north and west of their base, and the zerg will attack the player throughout the mission. The player is free to defend themselves without penalty, but destroying a zerg structure will result in the loss of the mission. The zerg will also attack from the south-west, and the Protoss ground forces attack from the south, and scouts will attack from the east over the mineral field. The player can wall off the northern part of their base to fend off the zerg easier, but will still need defenders there to stop the zerg from breaking the walls down.
The protoss attack with zealots and dragoons on the ground, and scouts in the air. Bunkers should be loaded with firebats and marines in a 2:2 ratio, the firebats being effective against the zerglings and zealots while siege tanks and marines handle ranged units. Dragoons and scouts are mechanical, making them fodder for Kerrigan's lockdown ability, and her normal attacks are effective against zealots. Missile turrets are also useful, as the player is vulnerable to aerial attacks from all angles. There is an expansion far to the south of the player's starting base, and it can be valuable if the player can defend it.
The player should quickly build up their defenses, and build three starports, as well as a science facility with the new physics lab add-on. This allows the player to construct battlecruisers and research their yamato gun ability. A fleet of a dozen of these ships will be enough to level both protoss bases, which lie in the north-east and south-east corners of the map. When attacking the player should use the yamato cannon to destroy photon cannons and scouts, the strongest resistance the player will encounter. Otherwise the protoss will defend with dragoons, but will not have enough to destroy the battlecruisers.
After one base is destroyed, the player can repair their battlecruisers and send them to destroy the other. The mission is won when all protoss structures are destroyed.