Mars City Underground: Union Aerospace Subsystems

"All units. This is Sergeant Kelly. We're under attack by an unknown enemy force. Fall back to Marine HQ to regroup! I say again fall back to Marine HQ and await further orders!"

- Sergeant Kelly

Mars City Underground: Union Aerospace Subsystems is the second level of Doom 3, composed of the main area and an abandoned communications building separated by a short walk outside. It is a small tutorial level covering a few more features in the game.

In the Communications Building, things start to go awry. The action begins here, and zombies are first seen. The Imp is introduced in a cutscene.

Summary
The Player arrives at the Mars City Underground, where he's greeted by T. Brooks, who gives him access to some basic gear: armor, a flashlight, and a pistol. The Player then heads through the facility to the old comm station where the missing scientist is believed to have gone. Along the way, he runs into several workers, all of whom seem to be nervous and on the edge. He also picks up Grant Baston's PDA, which contains a code for a locker containing supplies. After a brief walk across the Martian surface, he reaches the old comm station. Inside, he finds the missing scientist - Jonathan Ishii - trying to send a message to the UAC.

Ishii frantically tries to explain that he must send a warning out while there is still time. Before he can explain further, another teleportation experiment is conducted at the Delta Complex. Something goes horribly wrong, however. The portal stabilizers suddenly fail, causing a series of explosions throughout the base. At the same time, mysterious ghost-like skulls come flying out of the portal, possessing many workers and security guards. Ishii is possessed and transformed into a zombie, which attacks the player. After killing the zombified Ishii, along with a Z-Sec that bursts in moments later, the Player receives orders over the radio from Sergeant Kelly to fallback to Marine Command to regroup.

The Player must backtrack his way to the elevator at the beginning of the level. At one point, he is forced to take a detour through Engineering, and must use Frank Delahue's PDA to access the area. Along the way, he is attacked by zombies and Z-Secs, and eventually Imps. Fortunately, the Player also picks up a couple of better weapons - the shotgun and the machine gun. Throughout all this, the screams and panicked shouting of fellow marines can be heard over the radio. Sergeant Kelly is heard trying to coordinate a defense, but it's obvious the battle is going badly.

Eventually, the Player returns to the elevator and rides it back to Mars City.

PDAs

 * Daniel Young
 * Mark Ryan
 * Grant Baston
 * Frank Delahue

Characters

 * T. Brooks
 * Daniel Young
 * C. Rodgers
 * R. Fry
 * Ian Sinclair
 * Mark Ryan
 * Jonathan Ishii
 * Joe
 * UAC announcer
 * Sergeant Kelly
 * Bravo Team

Former Humans

 * Maintenance Zombie
 * Scientist Zombie
 * Jonathan Ishii
 * Ian Sinclair
 * Pistol Z-Sec
 * Shotgun Z-Sec
 * T. Brooks

Demons

 * Imp

Weapons/Items

 * Flashlight
 * Pistols
 * Shotguns
 * Machine Guns
 * Medkit
 * Security Armor
 * Armor Shards
 * Bullets
 * Shells
 * Clips
 * Adrenaline
 * Air Canisters
 * Plasma Cell

Cabinets

 * Cabinet #023: 531

Trivia

 * When you enter the "Energy Processing" room where you saw the scientist complaining to the maintenance guy earlier, after you kill the imp, there's a zombie hiding behind one of two pillars. Behind those pillars is a closet with shells and armor shards that never opens. The only way to see it is with the "noclip" cheat.