B.J. Blazkowicz IV is a space marine, one of Earth's toughest, hardened in combat and trained for action. Three years ago (before his transfer to Mars), he assaulted a superior officer for ordering his soldiers to fire upon civilians. The officer and his body cast were shipped to Pearl Harbor, while B.J. was transferred to Mars, home of the Union Aerospace Corporation.
In the original Doom, B.J. and his squad spent routine and boring life on Mars, with nothing going on. His day consisted of suckin' dust and watchin' restricted flicks in the rec room.
This changes when Mars received a garbled message from Phobos requesting immediate military support, and reports of something evil coming through gateways. Doomguy and his team were deployed quickly to Phobos. He was ordered to secure the perimeter of the base while the rest of his team went inside. He spent hours listening in on his radio to sounds of combat before things went silent. Assuming the worst for his buddies, he decided to investigate things on his own, and take revenge if necessary. Armed with only a pistol he entered the base hearing growls in the distance. Making his way through various facilities on Phobos defeating demons along the way, until he steps into a teleported and is transferred over to the lost moon of Deimos, now orbiting a hellish landscape.
Fighting his way through the facility he finally makes his way off Deimos to the surface of hell below. Fighting his way across demonic world he defeats another Cyberdemon (possibly the same demon his ancestor had defeated during World War II).
Doomguy ultimately uses a gateway to reach Earth, but finds it already ravaged by demons while he was in Hell, and his pet bunny, Daisy, decapitated. With a lucky rabbit's foot taken from Daisy upon him, Doomguy vowed to fight back.
Doom II/Final Doom
After the battle on Mars, Doomguy takes leave by drop pod and returns to Earth, only to discover it has been invaded by the monsters from the Outside. The survivors plan to flee the planet, but the remnant population of Earth and their ships are trapped on the starport controlled by a demon and the only remaining functional spaceport is under Hell's control
The Marine joins other marine troops to stop the invasion on Earth and deactivate the force field which contains the last spaceships. Since he had other Hell encounters in the past, the Marine is the only one who survives on the starport. He fights to take back the spaceport and the remaining humans succeed in fleeing to space. Doomguy then travels to the Demons' entry portal on Earth to close it and succeeds after killing the Icon of Sin. After he reaches a demon outpost, he manages to free the spaceships but becomes the only human left on Earth. After this, the Marine seems to accept his fate and wait for death.
After the spaceship's sensors locate the center of the invasion, which is located on the Marine's hometown, he abandons his suicidal waiting and goes there to stop Hell on Earth forever. However, after he reaches the main gateway, he learns that the only way to close it is on the other side, going to Hell again. This Hell is sightly different from the one on the original Doom since it has some demonic urban building - probably part of Earth inside Hell dimension. There, he discovers the Icon of Sin, the biggest demon he has ever seen, who is capable of creating demons and possibly the leader of Hell. After the destruction of the Icon of Sin, the Marine returns to Earth to help rebuild it, and Hell is left in ruins. Humanity then returns to Earth to rebuild.
The Plutonia Experiment
Following Doom II, the UAC finally found a way with the quantum accelerator to close all the seven gateways between Earth and Hell. But when the Gatekeeper, another Icon of Sin, leads another demon horde and manages to keep one gateway open, he began a full-scale Earth invasion. The Marine, who was on vacation, is called to contain the invasion on the UAC base and prevent another Hell on Earth. The Marine manages both to recover the quantum accelerator prototype and to destroy the Gatekeeper, which leads them to believe that Hell would be back as the place that only the bad people go when they die.
The Marine continued to work on the UAC, now as 'marine commander' of marine troops on one of Jupiter's moons. A year or more later, there, the UAC is conducting experiments to close the link between our universe and Hell, and after some successful tests, the base is invaded by an enormous demon ship, which invades the base and kills all personnel, with the exception of the Marine. He, once again, manages to go to Hell and destroys the Gatekeeper (another Icon of Sin).
He is a Corporal who has recently arrived on Mars as a replacement for "Corporal Allen", who was shipped back to Earth for psyche problems.
When he first lands everything appears to be normal, and he is given orders to find a missing scientist. After going through dungeon Mars Underground, and crossing over a short portion of the surface of Mars, he makes it into one of the older facilities and finds the scientist, who was attempting to send a message to earth warning about the research being conducted. But before he can send the message, Mars suffers from an incursion incident, with demons teleporting into the base and killing or possessing most of the staff, security, and marines.
Finally, the Corporal fights his way into Hell using the Soul Cube he defeats the Cyberdemon. Hell begins to harden, and he is teleported back to Mars. Soon after he is recovered by a contingent of Marines that were sent to look for survivors, and he walks out with them, again he is the only known survivor.
The Doom Marine (B.J. Blazkowicz IV), still employed by the UAC (after Doom I, II, Final Doom, and Doom 3), is transported to Mars from Earth. He is sent to assist members of the UAC Mars Outpost facility on Mars, who are under attack from the denizens of hell.
The Marine had been dispatched from Earth in response to a distress call from Union Aerospace Corporation's research facility on Mars. The base was under attack by an unknown force and his mission was to acquire intelligence and neutralize the threat. Once his insertion was completely ordered to rendezvous with the other Marines at Junction for further instructions. He was told to expect heavy resistance.
While their Marine meets Dr. Jensen who was accessing a computer terminal investigating his recent dismissal. During the exploration of the first few sectors of the installation, the Marine receives help from Dr. Guerard in gaining access to various locked down areas. In Biological Research Facility, the Marine meets Dr. Nadira who quarantines his weapons claiming security measures. Dr. Nadira then commands his mind-controlled hellhounds to attack the Marine and disappears in the process. Eventually, the Marine meets Jensen again. This time he is incarcerated in a prison cell. After freeing Jensen, he informs the Marine that Guerard and Nadira are into an evil scheme and directs the Marine to the next section of the installation.
The next time the Marine meets Guerard and Nadira, it is known that Guerard was behind the invasion and has been attempting to open a portal to Hell in the Reactor Sector. Guerard promptly orders demons to attack the Marine and Nadira. Nadira is killed by a Malwrath and Guerard escapes. The Marine proceeds through the remaining two sectors, acquiring the BFG 9000 and the key to the Reactor sector. By then, a major invasion devastates the Junction.
With the help of Kelvin and Jensen, the Marine gains access to the Reactor sector. Kelvin and Jensen both die in the process.
In the Reactor sector, Guerard reveals himself as Kronos and transforms into a demonic form. By this time, he has succeeded in opening the portal to Hell. After defeating Kronos, the Marine closes the portal by destroying the reactors powering it. The Cyberdemon slips through before the portal closes. The game ends with the defeat of the Cyberdemon. The Cyberdemon is likely to be Kronos' creation referred to throughout the base.
Doom 64/The Lost Levels
Many years later, the marine has been decommissioned, and UAC deep space stations have been quarantined and closed off and bombarded with radiation.
Something survived on Mars' moon, however. A satellite starts broadcasting a signal from the bases showing the Mother Demon resurrecting and leading the fallen armies of Hell. The Marine is recommissioned and sent to stop the horde before it can invade again.
The game's background story text mentions that he is, in fact, the same person from the original games, denoting him as "the only experienced survivor of the DOOM episode" and "the only Marine to endure the slaughter."
At the end of Doom 64, the Marine once again goes to Hell and manages to kill the Mother Demon. Unfortunately, he closes the portal from the inside and decides to remain in Hell to keep the demons from ever escaping it again. The Marine accepts his fate of eternal confrontation and prevents any other Hell invasion once and for all (or so it seems).