Doom 3 is set on Mars in 2145, where a military-industrial conglomerate has set up a scientific research facility into fields such as teleportation, biological research, and advanced weapons design. The teleportation experiments open a gateway to Hell, resulting in a catastrophic invasion of the Mars base by demons. The player controls a space marine who fights through the base to stop the demons attacking Mars and reaching Earth.
Doom 3 was a critical and commercial success; with more than 3.5 million copies of the game sold, it is the most successful game by developer id Software to date. Critics praised the game's graphics, presentation and atmosphere, although reviewers were divided by how close the gameplay was to that of the original Doom, focusing primarily on simply fighting through large numbers of enemy characters. The game was followed by Resurrection of Evil, an expansion pack developed by Nerve Software, in April 2005. A series of novelizations of Doom 3, written by Matthew J. Costello, debuted in February 2008. An expanded and remastered edition titled Doom 3: BFG Edition was released in the fourth quarter of 2012, while a version based on the BFG Edition of the game was released without online multiplayer, simply called Doom 3, for Android in June 2015, and Nintendo Switch, PlayStation 4 and Xbox One in July 2019.
Doom 3 is a story-driven action game played from a first-person perspective. As with previous Doom games, the main objective is to successfully pass through its levels, defeating a variety of enemy characters intent on killing the player's character. Doom 3's more story-centered approach, however, means that the player often encounters friendly non-player characters, who provide key plot information, objectives and inventory items. The game incorporates ten weapons for the players use to survive, including conventional firearms such as a submachine gun, shotgun and grenades, experimental plasma weaponry, and the powerful BFG 9000 and chainsaw weapons of the Doom franchise. Enemies come in multiple forms and with different abilities and tactics, but fall into two broad categories of either zombies or demons. Zombies are humans possessed by demonic forces, who attack the player's character using their hands and melee weapons or a variety of firearms, while demons are creatures from Hell, most of which attack using claws and spines, or by summoning plasma-based fireballs. The corpses of demons are reduced to ashes after death, leaving no trace of their body behind.
Frequent radio transmissions through the player's communications device also add to the atmosphere, by broadcasting certain sounds and messages from non-player characters meant to unsettle the player. Early in the game, during and directly after the event that plunges the base into chaos, the player often hears the sounds of fighting, screaming and dying through their radio transmitter. The ambient sound is extended to the base itself through such things as hissing pipes, footsteps, and occasional jarringly loud noises from machinery or other sources. Often ambient sounds can be heard that resemble deep breathing, unexplained voices and demonic taunting from the game's antagonists.
Doom 3 is set on the planet Mars in the year 2145. According to the game's backstory, the Union Aerospace Corporation (UAC) has grown to become the largest corporate entity in existence with near-unlimited funds, and has set up a research facility on Mars. At this base, the UAC are able to conduct research into several scientific areas, including advanced weapons development, biological research, space exploration and teleportation, outside of legal and moral boundaries. As the player progresses through the game, they learn that the employees on the base are unsettled due to a large number of incidents involving hearing voices, unexplained sightings and increasing cases of paranoia and insanity, often leading to fatal accidents with the facility's machinery. Rumors regarding the nature of experiments in the UAC's Delta Labs division are especially prevalent among the base's employees.
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 (Philip L. Clarke), the 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, and his demonic voice frequently taunts the player as the game progresses. The final two principal characters are Elliott Swann (Charles Dennis), a representative of the UAC's board of directors, and Jack Campbell (Andy Chanley), a space marine who acts as Swann's bodyguard and is armed with a BFG 9000. Swann is sent to Mars to check up on Betruger's research and investigate the rising number of accidents on the base after a request for assistance from a whistleblower. Swann is almost always accompanied by Campbell, and the two 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.
UAC board member Elliott Swann, his bodyguard Jack Campbell, and a recently transferred Marine arrive at Mars City, the main access to the UAC's Mars base. Sent to investigate multiple incidents involving the Delta Complex, Swann has a tense meeting with facility director Dr. Malcolm Betruger while the Marine reports to Master Sergeant Thomas Kelly for orders. Kelly orders the Marine to find a scientist from the Delta Labs who has gone missing. The marine finds the scientist in a decommissioned communications facility, where he is frantically trying to send a warning to the UAC on Earth about Betruger's teleportation experiments. However, as he tries to explain the situation to the marine, another teleportation test takes place but loses containment, at which point the entire Mars base is swept with a shockwave. The forces of Hell invade through the teleporter's portal and transform most of the base's personnel into zombies.
Now forced to fend off attacks from zombified base personnel and demons from Hell, the Marine returns to Mars City, where Kelly gives him orders to link up with another squad of marines and get a transmission card containing a distress call to the main communications facility to call for reinforcements. As the Marine progresses through the base, he learns that Swann and Campbell have survived, and are also en route to the communications facility to prevent any messages being sent in order to contain the situation on Mars. The marine squad is ambushed by demons and slaughtered in the EnPro Plant, and although the Marine recovers the transmission card, he is too late to prevent the bulk of equipment at the communications facility being destroyed by Campbell. However, Kelly directs the Marine to a backup system, where the Marine is given the choice of whether to obey Kelly's orders to send for reinforcements, or accept Swann's argument to keep Mars isolated until the nature of the invasion is understood, so as not to endanger Earth. The Marine is told to go to the Delta Labs by Kelly or Swann, depending on whether the transmission is sent or not.
On the way to the Delta Labs, the Marine is contacted by Betruger, who is now shown to be working in cooperation with Hell in order to invade Earth. If the Marine did not send the distress call to Earth, Betruger does so himself, hoping to use the ships bringing reinforcements to transport the demons to Earth. Betruger then unsuccessfully attempts to kill the Marine using the toxic gases in the base's recycling facilities. Upon arriving at the Delta Labs, the Marine learns of the details behind the teleportation experiments, expeditions into Hell to retrieve specimens and Betruger's increasing obsession with the tests, as well as of a xenoarchaeological dig under the surface of Mars. The dig is excavating the ruins of an ancient civilization discovered on Mars, and has produced a relic known as the Soul Cube. According to a scientist the Marine finds in the labs, the Soul Cube is a weapon created by the ancient civilization to defend against the forces of Hell. The scientist also reveals that the invasion began when Betruger took the Soul Cube into the portal at the beginning of the game, depositing it in Hell. The Marine pursues Betruger through the labs, but is pulled into the main teleportation portal after being lured into a trap by Betruger.
The portal takes the Marine directly into Hell, where he proceeds to fight his way through a large number of demons to the Soul Cube, defeating its demonic guardian. The Marine is then able to reinitialize the teleportation equipment left by previous research expeditions 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. The Marine encounters the injured Swann, who informs him 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 contai