In case you have never played it before there are three races. The Zerg, strange alien beasts who seem to want to over run everything. The Terran, basically humans with futuristic weaponry and armor. The Protoss, high tech aliens who claim to have been born to fight the zerg until the end.
Starcraft/broodwars- Raynor and Mengsk are in a rebel group to over through the Confederacy, an assassin (they call them ghosts) named Kerrigan is there too (Raynor is in luv with her). But Mengsk (who was the leader) went too far he put a device that attracts zerg and planted it on a confederate home world. For some reason the protoss who usually hate Terran decides to help fight of the Zerg so Mengsk orders Kerrigan to fight off the protoss long enough for the world to be overran. kerrigan does what is asked of her then calls for evac because her own forces are wiped out. Mengsk ignores her and leaves, the whole time raynor is protesting greatly. the zerg take kerrigan and turn her into the queen of blades some super psionic infected super zerg. So she has a personality change and helps the zerg fight, raynor helps the protoss fight zer and mengsk creates the dominain (just as bad as confederacy). the protoss home world is ran over and they must retreat to the dark templar homeworld. over confident the overmind plants itself into the protoss home world where a protoss hero will eventually take a ship and go kamakazie to take out the overmind.