Zeratul gathers the Protoss forces, battling rival factions, Terran pirates and/or remnants of Mengsk's armies, and the Zerg brood that Kerrigan conveniently forgot about. Along the way he gains allies, new units, and ridiculous upgrades like Colossi that can fly or some shit.
In the end there's an epic battle where he teams up with Kerrigan and Raynor to kill the evil guy.
That sounds fucking awesome.
Can you promise me there'll be terrible dialogue that sounds like it was written by a ninth grader putting together fanfiction for a creative writing class combined with being lifted from every SyFi channel B movie ever?