All life on the former fringe colonies of Mar Sara, Chau Sara and Antiga Prime had been extinguished when the Protoss rained fire down on the surface to eradicate the zerg infestation that the Terrans had suffered. With the downfall of the Confederacy and subsequent rise of the Dominion, these worlds remained largely ignored by Mengsk. 
He presumed, as others did, that they would remain uninhabitable.
However, among the blackened earth and desert wastes that covered the forgotten worlds, a small band of Terrans had returned to the homes they fled before the Armageddon engulfed their planets. Ever adaptable, these Terrans began to reconstruct their former cities and thrive once more. By keeping their settlements small and widely spread, surface activity had gone unnoticed by the Dominion and the Protoss. 

The first seven episodes in the campaign are available for download.

Episode 1 - Unwelcome guests
Ambush - A routine patrol goes horribly wrong - Download
The Swarm - You must prepare Mar Sara for the arrival of the Overmind  - Download
The Council - A Protoss fleet enters the Mar Sara system to invesitigate the zerg invasion  - Download

Episode 2 - Fugitives
In the Dark - Infiltrate the communications centre - Download
Nydus Island - A new addition to the swarm awaits - Download
Psi Matrix - Help Tutonas disable the network of Psi Cannons - Download

Episode 3 - Ad Astra
Evacuation -  Aid James Klim in his escape from Mar Sara- Download