Doom 3 levels

There are 27 levels in Doom 3. Most of them are quite large and typically require 1+ hours each on a player's first run through the game.

Spoiler Warning: Plot details follow.

1. Introduction (Mars City 1): A short tutorial on the basic features of the game. The player also picks up the first assignment from Sergeant Kelly ("Sarge").
2. Mars City Underground: A shorter tutorial covering a few more features in the game. In the Communications Building, things start to go awry. The shooting begins here. The imp is introduced at a cutscene.
3. Mars City (Mars City 2): The same level as the Introduction, but after the demon invasion.
4. UAC Administration: The pinky demon is introduced at a cutscene. The growth taking over the base is first seen, and this is where monsters start teleporting in.
5. Alpha Labs – Sector 1: Built in 2095 and located on Site 2, Alpha is the UAC's center research lab. Sector 1 houses the EPD (Elemental Phase Deconstructor) and the Hydrocon. Maggots are introduced.
6. Alpha Labs – Sector 2: Trites give their first appearance soon after the first web sighting. There is a glimpse of the Bravo Team through a window.
7. Alpha Labs – Sector 3: Contains a very hidden plasma gun and an optional chaingun.
8. MAP08: Alpha Labs – Sector 4: Player must navigate either the EFR or the bridge. There is the infamous "they took my baby" sequence, and the Vagary (first boss) appears at the end.
9. Energy Processing (EnPro): Bravo Team cutscene. The lost soul is introduced in a cutscene, and wraiths first appear here. Some rooms are less claustrophobic but more acrophobic, and there is an abundance of plasma cells. Swann and Campbell give up their attempt to intercept the marines sent to transmit a distress call, and therefore travel by vehicle to the communications complex.
10. Communications Transfer: There are several outdoor areas in this level. The first cacodemon appears at the start, and the chainsaw zombie makes its first appearances here. The berserk powerup appears for the first out of two times in the game.
11. Communications
12. Monorail Skybridge (Recycling – Sector 1): Where trash is processed in the UAC. The Revenant is introduced in a cutscene, and this is the only level with toxic waste pools.
13. Recycling – Sector 2: A trap is sprung on the player by Betruger, slowly filling the building with toxic gases. The mancubus is introduced in a cutscene and cherubs make their first appearances.
14. Monorail: Player must take it to reach the Delta Labs by monorail. Commando zombies first appear here.
15. Delta Labs – Level 1: This level contains no demon altercations of any kind for quite some time upon entering it. Player must initiate power by turning on main reactor.
16. Delta Labs – Level 2A: Player goes through the teleporter for the first time. The BFG9000 is first received.
17. Delta Labs – Level 2B: Arch-Viles begin to show up with their haunting cackles and screeches.
18. Delta Labs – Level 3: The player is hot on the heels of Betruger and travels through several second-generation teleporter units.
19. Delta Labs – Level 4: Hell Knights are introduced in the cutscene. Betruger sends the player to Hell through the main teleporter after the battle.
20. MAP20: Hell: The player has lost all weapons and ammo through the teleporter. The player must trek through Hell in order to defeat the Guardian (second boss) and retrieve the Soul Cube.
21. Delta Complex (Revisited): From hereon, there will be no more zombies other than commandos. The ticks make their first and only appearance.
22. Central Processing: The effects of the growth in the base are evident here.
23. MAP23: Central Processing (Server Banks): The player fights the third boss, Sabaoth, the demon transformation of Sergeant Kelly. Sabaoth has integrated Campbell's BFG into his cybernetic tank-body, and uses it against the player. After defeating Sabaoth, the player acquires Campbell's BFG.
24. Site 3: Where archaeological entities are brought to and where the research is done. This is the last level where the surface of Mars is visible.
25. Caverns – Level 1: The player advances towards the primary excavation site, where the Hell portal is located. Level 1 contains the oldest, original Mars base.
26. Caverns – Level 2: Features an ancient Martian temple. The Vagary makes another appearance at the end, and may come in a pair.
27. MAP27: Primary Excavation (Hellhole): The player must defeat the fourth and final boss, the Cyberdemon, in order to seal the Hell portal. An exploratory player may find the hidden "id Software" PDA, containing messages from several id Software employees.