Doom 3/Mars City Underground

When you enter the area, you will be stopped at the security checkpoint. After being told to collect your weapons and armor, you are let through the checkpoint.

Reaching the doctor
After going thorough check point, head down the stairs and proceed through the door. As you step down the stairs, a civilian will try to talk to you. Talk to him or ignore him.

In the next room, there is a civilian talking down a shaft with a discussion about

Dark room, use the flashlight.

After this room, M. Ryan will tell you that the scientist is through the airlock. Open it, and quickly reach the second airlock on the other side. (There are some items outside, such as adrenaline. It's not much use. )

After the second airlock, enter the elevator and reach the second floor. You can now proceed to your objective, but a cutscene will occur showing something going wrong.

Hell Breaks loose
The scientist will transform into a zombie; you can kill him immediately to prevent that. After a few seconds, the door will unlock, and a Pistol Z-Sec will approach and try to attack.

As you head out to the elevator, a zombie will try to approach from the side.

Head outside, and reach the other airlock. If desired, you can collect the two items on the left.

Mars City Undergound
After the second airlock, you are returning to Mars City. The inner airlock door will open only slightly while you see a zombie dragging a corpse. When it opens, head up the stairs and take a left. A zombie will try swinging a wrench at you as soon as you open the door.

Proceed to the maintainence bridge, deal with the zombie that climbs onto the platform, and extend the bridge. A pistol Z-Sec is waiting at the end of the bridge. If you are low on health, backpedal as soon as he is ready to fire to reduce the chance of taking damage.

Pas the bridge, you will enter a room containing a shotgun. Grabbing the shotgun will lower the floor with a four zombie ambush. Proceed through the upper door.

Past the upper door, the corridor will darken. Charge forward, and kill both the zombie, and Pistol Z-sec.

When you reach the platform that turns to the left, an imp will wait just behind the door. You will also find a cabinet that has the code 531, but you will want to watch for a Z-Sec sneaking up from behind.

Exiting underground
An imp will approach from where you first saw the two workers arguing.

When you reach the staircase with the person that warned you, an imp will burst out of the stairs.

Head into the main chamber. Amn imp will climb the side and attack. A second imp is in the security office. Enter and unlock the exit, killing the Z-Sec that awakens. You can also unlock any desired weapon lockers, allowing you to collect the machine gun.

The main exit elevator has a zombie, but you can bypass it simply by starting the transfer.