You play in a room with Barney, Dog, Alyx, Father Grigori, a vortigaunt, a few citizens and the G-man. The room is just a box with a large image of Dr. Breen on the wall and a bunch of zombie trying to kill everything.
You are Frank Harrison, a rebel, who lives near City 17.
You wake up and find that the Citadel looks like it is about to explode and those scientists are urging you to flee the city.
No need to tell you twice! Off you go and good luck – you’ll need it.