Half-Life: Opposing Force/Welcome to Black Mesa

The introduction is rather short, where you are in transit to the Black Mesa facility. However, before you are given your orders, the transport chopper is shot down by alien forces. After crashing into the middle of the facility, a second airstrike incapacitates you.

Black Mesa
You awaken near a scientist that managed to revive you, but couldn't help the other two soldiers. He recommends using the radio located near the crash site to call for help.

Exit the room, and when you come to a junction, take a left and collect your PCV. Head down the other corridor and through the door, and ask Otis to open the door for you.

Behind the door, you will need to collect a wrench near the electric wire. When you are ready to continue to the next room, you will see a laser beam blocking the route up the stairs. Look for the mirror that's reflecting the purple beam, and smash it to continue.

Head crabs drop from the ceiling in the next corridors. Head up the stairs to the doorway leading outside.

Outside cliff
Head outside, and jump down to the ground below the bridge. Loop around and head up to the concrete and jump across. Enter the building and then hole in the back-right.

The airplane is right against an electric fence. As such, you have to enter the tunnel to the right. You should get to a generator occupied by Otis, who then tries climbing the electrified fencing. To turn it off, head into the ductwork to the right, and circle around to the right of the sparking generator, to enter the other duct. You should reach the main generator area. Avoid the electrical beams to reach the switch in the corner. Turn it off.

Return to the airplane; two alien slaves will teleport in. The radio will tell you that there are orders to pull out, and to go through the transit system.

Head down the ladder to the left and on the sublevel, exit to the left. Watch for barnacles on the ceiling. Head through the two doors.

Slime works
Get the shotgun from the dead soldier, and go down the ladder. Enter the door on the lowest floor, and turn left when you see the lift. Jump across the broken bridge, and when you reach the other side of the room, a high-voltage generator will start blasting the floor; stay away from it.

Eventually, it will knock down a floor on the upper portion - climb on it before the slime starts rising. The G-Man to the left will open the door for you just before as you approach the door. Head down the lift.

Transit
In the next room, you see that slime has leaked through the secure access door. Jump over it by the conveyor and head into the visible control room (by the floor or window). Move the conveyor to the left of the control room window, then use it to jump across to the door switch on the left. Open the door, jump back onto the lift, then onto the metal box on the right of the door. Jump off.

You should reach the defunct transit system, but to exit, you have to enter a broken pipe in the back area.

Sewers
Swim through the sewers and go up the ladder. The catwalk won't collapse, thus head into the sewage overflow control.

A robotic loader is trying to operate, but is blocked by boxes. Shoot an explosives box to clear the path. Head down the ladder, jump across the boxes and start the pump to clear the area.

Head up the lift. Move the trolly to the left, and climb aboard the tram.