1. Why don't you plan yourself?
2. Shephard, together with a small task force (4-5 troopers?) infiltrates Black Mesa by night. Their goal is to set up a small communication post and scout out the area before the others will arrive. Black Mesa is already infested at that moment and when they enter trough a small water filtering system, their team is being attacked by shock troopers. During the fight, Shephard falls into the water and the screen fades black. Later, he awakens with 2 of his fellow troopers trying to cure him. They've set up a radio and they've asked for extraction, but HQ told them that may take some time.
During this talk, shock troopers invade the room and kill your mates. The screen fades black again. (the shock troopers think you're dead so they leave the room without harming you) Later, you awake and it's up to you to find a way out to the extraction point.
Well, maybe that helps a bit... throw in a nest of those pitbeasts and such and you've got yourself some little survival story...