Shepard climbs his way out of the Waste Processing area back to the surface only to run into the Black Ops again. The corporal fights his way through, and manages to come across several stranded HECU soldiers pinned down by the Black Ops sniper in the area. With the support of his new squad, Shepard continues his fight through the Black Ops, which has also erupted into a fierce battle against the aliens of Race X.
The corporal eventually reaches an entrance that drops him down into a sewer network that turned into a nest for the Voltigores. Shepard fights through the Voltigore nest, and comes out the sewer network on the other side where several other HECU soldiers have taken shelter. The corporal and his new squad fight through the next area that's been completely taken over by Race X's Shock Troopers and Voltigores. They make it to Black Mesa's hydroelectric dam where several other HECU soldiers are attempting to kill a Gargantua they've pinned down on top of it. Shepard activates the nearby explosives to kill the Gargantua, which also results in a hole being blown into the dam that leaves its pipes exposed. Shepard then jumps into the dam pipe, and rides through it to reach the next area.
This Chapter contains examples of:
- Blackout Basement: Half-way through the level, Shephard is forced to go through a pitch black sewer filled with extremely difficult enemies.
- Call-Back: Shepard arrives at the hydroelectric dam that was seen in the original game's "Surface Tension".
- The Engineer: One of the squad mates for the area after the Voltigore infested sewer is an Engineer. There's no doors for him to open in this area though. His only purpose is a scripted event where he opens up the floor grate that's keeping Shepard trapped in the sewer.
- Evil Versus Evil: From this level onward, both the Black Ops and Race X aliens can be seen fighting one another regularly. And both will still try to go after Shephard and the other HECU Marines should they get near.
- From Bad to Worse: Even with the Nihilanth dead and the Xen invasion petering out, things in Black Mesa have gotten even worse than when Freeman left for Xen. The Race X aliens have stepped up their own invasion, and the Black Ops have begun sending in reinforcements to continue their mission of eliminating both the Alien Invasion and any surviving witnesses.
- Make Way for the New Villains: While it was already becoming kind of obvious in the previous chapters, this one makes it quite clear that the Xen aliens have been phased out in favor of the Black Ops and Race X aliens being the main antagonists. Justified Trope, on account that this level takes place soon after Gordon Freeman kills the Nihilanth, thus cutting Xen off from Earth.
- Meaningful Name: "Foxtrot Uniform" in NATO alphabet is "F" and "U." In this context they mean "Fucked up". Which is what the situation at Black Mesa most certainly is.
- The Medic: One of the squad mates for the area after the Voltigore infested sewer is a Medic.
- Mêlée à Trois: HECU vs. Black Ops vs. Race X.
- Remixed Level: Early on you navigate through a compound with many containers and explosive traps deployed. Thankfully you don't have to maneuver too much around them but you need to avoid setting them off as you fight the Black Ops soldiers in the area. When you're directed into the sewers you can hear bombs go off on the surface, and upon reaching the end, you end up back in the same place as before, only reduced to ruins, full of Race X aliens, and a door to the real next area is conveniently blasted open and you need to get there.
- Series Continuity Error: An oversight from the Opposing Force developers is that the area behind the door of the hydroelectric dam is completely different compared to what it was within the original Half-Life.
- In Half-Life, the area beyond the dam's door is a straight outdoor path after coming out of the sewer in the "Forget About Freeman!" level. The path is guarded by a tank blocking the elevator that leads down to the Lambda sector.
- in Opposing Force the area beyond the dam's door is no longer a path leading to the Lambda sector, but is now a large storage room.
