You are a changeling from a small brood of Zerg that was just getting settled on a random planet. The Protoss noticed you and, paranoid after the loss of Aiur, took you out immediately. One overseer however, had been scouting. Though doomed by Photon Cannons, the overseer gave birth to a Changeling before his death. Now you are all that's left. Master the art of shapeshifting and wipe out the Protoss (and Terran) forces on the planet. Avenge your brood!
You learn to shapeshift as almost every unit in the game, not simply a zealot/marine/zergling. Each unit has 2 or 3 skills that can be mixed to form tons of combos. Ex: Ghost Snipe -> Zealot Rush -> Ultralisk Sweeping Attack -> Stalker Blink backwards -> Marauder slow attack -> Kite