Part 4

Leave the break room and follow the green wall stripe to the employee locker room.

Enter the bathroom in the back right corner, then proceed to the HEV suit room in the back left corner.

Use the control panel in the HEV suit room to open a case with a suit inside, then walk into the case to put it on.

Find the locker labeled "Freeman" near the bathroom entrance and open its door.

Enter the "Freeman" locker to pick up an armor battery.

Check the wall-mounted first aid station around the corner. (Note: First aid stations won't make their characteristic sound if your health is 100.)

Return to the main corridor and follow the blue wall stripe to the security post.

The guard will open the security door for you once they see you wearing the HEV suit.

Proceed through the security door and the next door to load section c1a0a.

In section c1a0a, use the button panel to the left of some doors to open them.

Get on the lift past the doors and use its button to descend to a lower level (or climb down the side ladder).

Go through some doors and down a corridor until section c1a0b loads.

In section c1a0b, enter the control room and wait for three scientists to talk to you.

After one scientist uses a retinal scanner, go through the unlocked door.

Across from two scientists who notice equipment problems, use a button panel to summon a lift.

Use the lift's button panel to go up a level.

Exit the lift and go down the corridor until you reach the test chamber door, loading section c1a0e.

In section c1a0e, enter the test chamber's outer room and wait for two scientists to open the main test chamber doors.

Go through the main doors and across the chamber to a small ramp.

Climb the ladder past the ramp to reach an upper walkway.

Use the walkway's computer panel and press the button when its red glass cover (press the cover button to crouch behind walls or objects) rises.

Watch as the scanning machine powers up.

Climb back down the ladder and stand next to the cage near the ramp's bottom.

When the "specimen" arrives, quicksave.

Get behind the gadget the specimen is mounted on and push it into the scanning beam.

Move back and wait to be teleported to border world locations, leading to the "Unforeseen Consequences" chapter.

In section c1a0c, check for a rare vortigaunt encounter in the test chamber ruins. If one appears, restore your quicksave and repeat the teleportation process. (Avoid pulling the specimen's gadget out of the scanning beam after insertion, as this almost certainly triggers the vortigaunt.)

Leave the test chamber through the jammed-open doors.

Try using the retinal scan device next to the exit door.

After the door opens, go through it to find a scientist reviving a security guard. (The guard cannot overcome nearby rubble, so taking them is not helpful.)

Go around the corner and move forward slowly.

After a computer bank falls over, go past it and jump over the debris in the corridor leading to the lift.

Use the lift and go through the doors.

Approach the two scientists and have the black scientist (Dr. Vance) follow you to the control room door's retinal scanner.

After Dr. Vance opens the door, move far enough inside to see a large electrical surge shoot into the control room through the window on the right.

Wait for the surge to blow up the door at the other end of the room.

Carefully time your run across the room to avoid being zapped.

In the next hallway, section c1a1 will load.

In section c1a1, run through the next room and out its other door quickly, as a headcrab will teleport in.

In the next hallway, crouch-walk under the first red laser beam.

Run around the second laser beam after it breaks free from its pipe, near a dead security guard.

Pick up the crowbar next to the nearby doors.

Return to the room where the headcrab teleported in.

Quicksave before entering the room.

If the headcrab is levitating, go around the fallen pillar, approach the headcrab, and right-strafe at the last moment to dodge its leap.

Quickly turn, run up to the headcrab, crouch, and use the crowbar to attack it before it can leap again.