
The marine is then able to reinitialize the teleportation equipment left by research expeditions into Hell and return to the Delta Labs. 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 pursues Betruger through the labs, but is pulled into the main teleportation portal after being lured into a trap by Betruger. 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.

According to a scientist the marine finds alive in the labs, the Soul Cube is a weapon created by the ancient civilization to defend against the forces of Hell. The dig is excavating the ruins of an ancient civilization discovered on Mars, and has produced a relic known as the Soul Cube.

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 an archaeological dig under the surface of Mars. Betruger then unsuccessfully attempts to kill the marine using the toxic gases in the base's recycling facilities. 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. On the way to the Delta Labs, the marine is contacted by Betruger, who is now clearly shown to be working in cooperation with Hell in order to invade Earth. The marine is told to go to the Delta Labs by Kelly or Swann, depending on whether the transmission is sent or not. Kelly, however, 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 exact nature of the invasion is understood, so as not to endanger Earth. 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. 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 hope of containing the situation on Mars.

Now forced to fend off attacks from zombified base personnel and the demons from Hell, the marine returns to Mars City, where Kelly remotely gives the marine orders to link up with another squad of marines (Bravo Team) and get a transmission card containing a distress call to the main communications facility to call for reinforcements.

However, as he tries to explain the situation to the marine, another teleportation test takes place and loses containment, at which point the entire Mars base is swept with an unnatural shockwave, transforming most of the base's personnel into zombies as the forces of Hell invade through the teleporter's portal. The marine finds the scientist in a nearby decommissioned communications facility, where he is frantically trying to send a warning to the UAC on Earth about Betruger's teleportation experiments. Kelly gives the marine instructions to find a scientist from the Delta Labs who has gone missing. Malcolm Betruger as the first part of investigating the incidents and complaints about the facility, while the marine heads to Master Sergeant Thomas Kelly for orders. Swann and Campbell have a heated conference meeting with Dr. The game opens with Elliott Swann and Jack Campbell arriving at Mars City, the main access to the UAC's Mars base, disembarking from an Earth transport, with the player's anonymous marine just behind them. The story of Doom 3 is conveyed through in-game dialogue and cut scenes, as well as e-mails, audio logs and video files found throughout the game.
