The portal takes the marine directly into Hell, where he proceeds to fight his way through the large number of demons to the Soul Cube, defeating its demonic guardian. The marine is then able to reinitialize the teleportation equipment left by research expeditions into Hell and return to the Delta Labs. Betruger, however, tells the marine that although the main UAC teleporter has been destroyed, Hell is opening a Hellmouth on Mars, capable of bringing millions of demons to Mars.Further in the De
lta Labs, the marine encounters the injured Swann. Swann informs the marine that Kelly has been working with Hell for possibly the whole time, and has been transformed by the demons. Telling the marine that Campbell has gone after Kelly, Swann gives the marine his PDA containing information on the location on the Hellmouth under the surface of Mars and assures him that he will try to make his way out of the base alone.
However, when the marine catches up with Campbell in the central computer processing sector of the base, Campbell is mortally wounded and only has enough strength to say that Kelly has taken Campbell's BFG 9000 weapon before expiring. Kelly then begins to taunt the marine in a demonic voice. The marine eventually faces off with Kelly in the central computer core, revealing Kelly as a cybernetic human grafted onto a tank-like base. The marine is able to kill Kelly and takes the BFG 9000 before proceeding deeper under the Martian surface to Site 3, the archaeological dig site where the Soul Cube was unearthed. At the primary excavation site, the marine discovers the Hellmouth, defended by Hell's mightiest warrior, the Cyberdemon. Using the Soul Cube, the marine defeats the Cyberdemon in combat, and the Soul Cube then seals the Hellmouth. The ending cut scene shows the reinforcements from Earth arriving at the base to discover the carnage. They find the marine alive, but discover that Swann has died. They are, however, unable to locate Betruger, who in the final scene is shown in Hell, reincarnated as a dragon-like demon.
There are five main characters in Doom 3. The player assumes the role of an anonymous space marine corporal who has just arrived on the UAC's Mars base. The player's non-commissioned officer in-charge is Master Sergeant Thomas Kelly, voiced by Neil Ross, who gives the player objectives and advice over the player's radio for the first half of the game. The antagonist in the story is Dr. Malcolm Betruger, head scientist of the UAC's enigmatic Delta Labs division, who is revealed to be working in collaboration with the forces of Hell to cause the subjugation of humanity. Betruger's demonic voice, provided by Philip L. Clarke, frequently taunts the player as the game progresses. The final two principal characters are Elliott Swann and Jack Campbell. Swann, voiced by Charles Dennis, is a representative of the UAC's board of directors, sent to check up on Betruger's research as well as investigate the rising number of accidents on the Mars base after a request for assistance from a whistleblower. He is almost always accompanied by Campbell, another space marine who acts as Swann'sbodyguard and is armed with a BFG 9000. Campbell is voiced by Andy Chanley. Swann and Campbell are often shown in the game to be a few steps ahead of the player, but cannot be reached and directly communicated with until late in the game. The game also incorporates a large host of minor characters who add details to the story or assist the player in certain segments. The player encounters multiple scientists involved in the various research and development programs and archaeological digs through the UAC base, as well as fellow marines and security guards. Civilian employees engaged in bureaucratic work and maintenance workers are also seen.