Following the escape of Commander Jim Raynor and Emperor Arcturus Mengsk from Aiur, the UED Expeditionary Fleet was forced to flee the planet to escape overwhelming zerg forces. During the battle, Vice Admiral Alexei Stukov went AWOL with a substantial number of UED troops, and returned to the previously conquered planet of Braxis. Lieutenant Samir Duran suggested to Admiral Gerard DuGalle that Stukov may not have been entirely loyal, angering DuGalle due to his long friendship with Stukov and their service record together.
At this moment, the UED adjutant informed DuGalle that their sensors had found a functional psi disrupter on Braxis; it seemed that Stukov had not destroyed the disrupter when they found it on Tarsonis, but moved it here. Duran declared this was proof Stukov was a traitor, as he had abandoned the UED and retained the disrupter to sabotage their mission. DuGalle was aghast, but could not refute the accusations. He ordered Duran to lead a strike team into the psi disrupter and execute Stukov.
Duran infiltrated the disrupter, guarded by Stukov's Elite Guard. He and the UED fought their way through the opposition to Stukov, where Duran shot him fatally and then cloaked and vanished. The dying Stukov contacted DuGalle and revealed that it was Duran that was the traitor, allowing the zerg to overtake them on Aiur so Raynor and Mengsk could escape, and convincing DuGalle the disrupter should be destroyed when it was the UED's best hope of victory. Stukov even suspected Duran might be infested and aiding the zerg. He urged DuGalle to use the disrupter and finish their mission, and fell dead.
The distraught DuGalle demanded to know what had happened to Duran, and the adjutant informed DuGalle that Duran was no longer appearing on her sensors. At that moment, the psi disrupter's power core was set to overload, requiring a manual override to stop before it exploded. DuGalle contacted the remnants of Stukov's guards and asked them to continue to serve Stukov's memory, by aiding the UED force in saving the disrupter. As the reunited UED forces explore the rest of the disrupter, numerous zerg broke into the disrupter and attacked them.
Ultimately, the mission was successful; the overload was halted and the disrupter saved. DuGalle ordered the disrupter kept on full power around the clock, and stationed a garrison on Braxis to defend and maintain it. With this done, the bulk of the UED fleet departed for Char to claim their ultimate objective; the subjugation of the second Overmind.
The opening scene has nine marines shoot their way into the disrupter; all nine should survive, if not the player can restart the mission, since their positioning is random and one may die in the attack. The player then receives control of Duran and two medics. To the south is a computer beacon, but accessing it will just spawn two sentry guns. Moving to the north-east hallway and following it, the player will find a short scene with some civilians, after which more marines move in and more sentry guns activate by the door. In the north room is another computer, which reveals a goliath repair bay further ahead. Further north is a small room with nothing but two firebats to kill.
Moving south the player finds more enemy marines and a computer, which triggers a small scene with some civilians and then reveals the location of Stukov deeper into the installation. From here the player enters the goliath repair bay. Moving a marine to a goliath will remove the marine and commandeer the goliath, with there being six goliaths total. The player may wish to consider passing up the goliaths; they will find a SCV shortly that can repair them, but only has limited resources to do so, while the marines have medics with infinite healing. Either way, enemy civilians will come to board the goliaths, so the player should kill them as they come down the hall before they do. In the next room the player will find the SCV, who comes under their control with 150 minerals and gas for repairs.
Onward are several enemy marines and a goliath, and a ghost on high ground. At this point, the player should cloak Duran and move him on alone. A missile trap on low ground will detect Duran, but it can be hit with lockdown to bypass the firebats nearby. At the doorway above, three turrets will attack; if the player approaches with Duran from the lower-right corner of the door, one turret will pop up and can be killed, allowing Duran to pass by the other two, taking fire but surviving. With Duran cloaked to avoid some marines, the player can move him to the beacon and confront Stukov.
After Stukov dies and Duran vanishes, Stukov's guards are set to rescuable, and most of the automatic defenses are disabled. The remainder of the mission has fifteen minutes to complete, plenty of time as long as the player doesn't dally. The player can use their forces to recruit the firebats, marines, the ghost and the goliath Duran passed previously, as well as some marines that enter the door behind where Stukov was. Near them a computer reveals the location of the override terminal. When the player approaches the stairs here, a wave of zerglings attack, the marines can easily hold them off from the high ground. Up the next flight of stairs are more zerglings, and to the north a room with more marines and firebats.
So the south-east a computer terminal reveals a massive zergling swarm up ahead. Near the terminal are more zerglings, and some will attack the player when they move down the hallway ahead. To the south is a small room with some marines and medics. Down the hall the player will find ultralisks on low ground, and can kill them. However, more ultralisks will soon attack from the north. The player should station their goliaths in front to take their attacks instead of the more fragile marines. Onward in a long hallway, the player will activate friendly sentry guns. The zergling swarm up ahead can be lured into the guns and the guns will destroy them with no losses to the player's forces.
Past the zerglings several hydralisks attack. Down the next hall infested terrans will attack, stimmed marines should be able to kill them before they can explode. Ahead are more zerglings, and a large number of marines holding off a swarm of zerg from high ground. Any survivors of the attack join the player. In a humorous scene, to the north in a small alcove is another marine hiding from the fight. Down the lower hall a massive zergling swarm will attack, the player's army should be quite large now and can handle them. The player should move through the next room cautiously; a defiler will unburrow and cast plague, the medics should quickly restore it and heal the player's forces. Beyond here are several more groups of burrowed zerglings, and then the override terminal. When the player reaches the terminal the mission is won.
There is one missile trap that will attack Stukov's forces (if the player manages to aggro them to the trap) and will continue to attack the player after Stukov dies and Duran vanishes. It is located after several enemy marines and a goliath, and before the room with missile trap on low ground and firebats.
If the player manages the get back to the beginning of the mission after the zerg presence being discovered, there will be four white marines that can be rescued. However, zerg will spawn on the route back as well.