Who says you'll be playing as Adrain Shephard? He might've gone off and died, who cares.
You start out as a combine metrocop - harrassing the normal citizen, throwing coke cans at old people.. but then, they find Gordon and you get called in to become a combine soldier. Boom, you get all these implants and stuff (Bullettime, anyone?) and 'tis time to kick some rebel ass.