Continuing towards the extraction point, Shepard rides the tram fighting off any Xen aliens he comes across until getting off at the next station. The corporal then makes his way up to the station's viewing room only to witness the first appearance of an alien from "Race X;" a new faction of aliens separate from Xen that were also attracted to Earth through the Resonance Cascade. Said alien, known as a Shock Trooper, kills the Black Mesa security guard in the room, and then grabs the nearby scientist to teleport them both back to Race X's homeworld; supposedly for Race X to study humanity.
After a bit more trekking through the facility, Shepard eventually reaches the extraction point where the last few HECU soldiers in the area are seen getting onto the Osprey before it takes off. Shepard heads for the Osprey, but the exit door shuts on him due to the G-man's interference; resulting in the Osprey taking off and leaving Shepard behind. Xen aliens then burst through the wall behind Shepard to attack him, which Shepard uses the newly opened path to try to find another means of escaping the facility.
This Chapter contains examples of:
- Call-Back: The "Forget about Freeman" radio call turns up in this expansion, except this time, we get to meet the guy making the call. At the Osprey extraction point, Shepard comes across the soldier who radioed to Cooper about bombing targets on the tactical map in the original game's "Surface Tension." It's then revealed that the soldier in question in a high-ranking officer, and one of Shephard's superiors.
- Early-Bird Cameo: "Race X" makes its debut here as it turns out that the Xen aliens were not the only alien faction that the Resonance Cascade attracted to Earth. As the game goes on, the aliens of Race X phase out the Xen aliens until there's only Race X aliens remaining within Black Mesa. To this day, Race X remains exclusive to Opposing Force.
- Helpless Window Death: The first Shock Trooper is shown teleporting into a locked room with a guard and scientist inside. Shephard can only watch as it kills the guard and abducts the scientist, the latter of which breaks the window that was blocking him from intervening.
- Missed the Bus: Shepard attempts go down and board the last Osprey out of Black Mesa only to have the door shut in his face.
- Vengeful Vending Machine: Early on, Shephard encounters a fat security guard named Otis with his hand stuck in a vending machine. The guard curses, with lines like "Stupid machine!" and asks the player for a quarter. It's possible to operate the machine with the Use key, and this makes a candy bar pop out. However, the guard does nothing if you do so.
