In the original Doom
, he's a nameless marine sent to do grunt work on Mars after an incident over not wanting to kill innocent people on Earth causes him to lash out against his commanding officer. During his dull-as-dirt assignment
, it seems the scientists of the Union Aerospace Corporation (UAC) are conducting experiments with teleporters when everything falls to pieces. One of Mars' moons and a UAC outpost, Deimos, completely vanishes into the ether while another, Phobos, is immediately overrun by The Legions of Hell
. Everyone is either slaughtered or converted into minions of the invaders. The marine dispatches to Phobos, where he proceeds to clean up the situation as brutally as possible. During his one-man war against the forces of Hell, he discovers what became of Deimos (ended up floating above Hell itself) and then treks down to the surface of Hell to finish the job.
When the sequel, Doom II: Hell on Earth
, rolls around, the marine had just finished ripping Hell a new one when a distress signal alerts him that the armies that invaded the Mars moon bases have now breached their way to Earth. The humans who are able to escape the genocide decide their only recourse is to escape the planet but their exit route is barricaded by the invaders and obstructed by a force field they set up. The marine is called to break the line of defense and deactivate the force field on behalf of the other humans so they may escape, which he does and remains as the last human on Earth. However, further communication from the escaped humans reveals that the epicenter of the invasion is still on Earth, allowing the marine to finally push back against the invaders. After fighting their forces he decides to cross back into Hell yet again, this time with the intent of making sure they can never invade anywhere ever again. After this trek through Hell, he manages to essentially destroy
it, literally leaving no place for the damned to go once they die. Come Doom 64, Earth is finally mopping up from the attacks from the first two games, when something resurrects all the demons on Phobos and Deimos, prompting another assault that ends in the marine storming Hell a third time and apparently wiping demons off the face of Hell completely by killing the Mother Demon.
In Doom 3
, since it serves as a Continuity Reboot
, Doomguy is a whole new marine, who simply happens to have the bad luck of being transferred to the UAC's Mars base the day Betruger unleashes hell upon the planet. Through determination and sheer luck, he manages to stop the invasion and close the portals, trapping Betruger in Hell after slaying Hell's ultimate warrior, the Cyberdemon.
In Doom 3: Resurrection of Evil, yet another new marine takes up the mantle of Doomguy, this time a marine exploring Mars as part of an expedition to try and figure out just what Betruger actually did. Discovering a demonic Artefact that Betruger attempts to use to generate a second invasion, he fights his way into Hell and slays Betruger, stopping his plans cold.