Half-Life Opposing Force-Friendly Fire

Adrian Shephard and other HECU soldiers are attempting to find and use an old transit system to escape to the Lambda Core after battling through an office complex filled with Vortigaunts. Shephard comes across the Black Ops in a hangar where they are loading a nuclear warhead into the back of a truck. This is where he learns that the Black Ops soldiers have been sent to kill the remaining HECU Marines and cover up the Black Mesa incident. Shephard meets up with several other surviving marines and fights the Black Ops, eventually reaching an underground railway system that leads to the Lambda complex.