Chapter by Chapter:
Insecurity: It's Barney Calhoun's first day on the job as a security officer. He must get his uniform and meet up with some scientists who will lead him down Black Mesa. However, during their travel down, the resonance cascade occurs and Calhoun is knocked unconscious.
Duty Calls: Calhoun wakes up and sees that the elevator is destroyed and he needs to find help. He travels through a water disposal area and a storage facility during his journey.
Captive Freight: Calhoun discovers the marines. He needs to fight his way through a military infested cargo hold and search for Dr. Rosenberg.
Focal Point: Dr. Rosenberg teleports Calhoun to Xen so he can reroute a communications tower to re-establish contact with Rosenberg.
Power Struggle: Calhoun goes underground to a coolant facility where he needs to bring a power cell up to Rosenberg.
A Leap of Faith: Rosenberg creates a portal that Calhoun must go through along with scientists. The marines cannot stop him.