AwakenedAt the beginning, the Doom Slayer awakens naked in a strange room that is infested with possessed humans. After clearing the room and picking up an energy pistol, he tries to leave but the hologram of a strange, sickly-looking woman appears when he touches the door control. She walks over to the slab where the Slayer was chained, now showing a holographic depictions of the completed sarcophagus and several followers bowing down before the hologram terminates. In the next room, he reclaims his Praetor Suit and then consults the nearby panel for more information on his current situation. However, due to malfunctions in the satellite array of the facility he's in, the Slayer is unable to find any usable information other than the source of the malfunction. At that moment, a voice call breaks through and a robotic voice on the other end introduces himself as "Dr. Samuel Hayden" and claims to be in charge of the facility. He proposes a working arrangement but the Doom Slayer shoves the panel aside instead of hearing out the unknown individual.
The Doom Slayer fights his way through past possessed humans and demonic monsters until he reaches the elevator to the surface, encountering another hologram of the mysterious woman and claiming a shotgun along the way. As he rides the elevator up, the voice of Samuel Hayden once again comes over the comms system and attempts to reassure the Slayer that, while what he is witnessing was not his intention, the work that has caused this situation still holds valuable merit. The Slayer, having no patience for the flimsy justification for all the horrors that have taken place, smashes the comms system before he can finish. At the surface, the Slayer surveys the dusty Martian landscape before venturing forth and fighting more hordes as he treks across the alien world. He reaches an airlock that lets him inside the facility, learning from a nearby hologram that the site is owned by a group called "the UAC".
Passing through the airlock, another voice comes over the communication system, this time a pleasant voice that introduces itself as an A.I. named "VEGA". Within the facility, the Doom Slayer finds himself unable to proceed until he reactivates the power to the area. Locating a power breaker, he reactivates the power but finds that the satellite is still out of alignment. Along with the power reactivation, a prerecorded video of the frail woman from the earlier holograms appears on the nearby panel to offer some ominous words, and the video identifies her as "Olivia Pierce". The Slayer continues to clear the facility of demons and on the surface beyond before finally coming across the satellite control deck. He reactivates the satellite but discovers he still can't get any information, as Hayden has blocked the system from giving the Slayer any data, telling him to go to the VEGA terminal in order to get what he's looking for.
Pushing deeper into the facility at Hayden's insistence, the Doom Slayer arrives at the VEGA terminal and is presented with an orb described as "pure Argent energy" by Samuel Hayden, which gives the Slayer a boost of power. Hayden instructs the VEGA A.I. to give the Slayer the data he was trying to access at the satellite control deck, providing a full picture of the destruction wrought upon the UAC by the invasion. Apparently Olivia Pierce unleashed a massive energy wave that converted the employees of the UAC into the hellish monsters roaming around and those who did not get transformed were instead slaughtered by captive demons that Pierce released. Hayden concedes that Pierce went too far but once again reinforces his own position that the work the UAC has done is a necessary service by tapping into the dimension of Hell. Before Hayden can continue, VEGA issues a warning that the nearby foundry is in danger of melting down, which Hayden agrees needs to be made a priority for the sake of both his own and the Slayer's survival.
Venture to HellAt the foundry, Hayden informs the Doom Slayer that the security protocols in place prevent any sort of tampering with the foundry's reactors before the demonic presence is reduced, something he himself is unable to override. The demonic threat in the foundry is primarily originating from the presence of four "Gore Nests" that the Slayer diligently eliminates as he explores the area. Much of the facility is under authorization-restricted lockdowns, forcing the Slayer to use keycards stolen off bodies and even the fingerprint off the arm of a slain UAC employee to progress to stopping the meltdown. With the area clear of demons, the path to the turbine room is unlocked and the critical heat within the turbines is vented, preventing the meltdown. After fighting through another wave of demons attack the turbine room, Dr. Hayden notes that a control room nearby has experienced some activity, which he theorizes could be Olivia Pierce.
A short distance away, the Doom Slayer encounters a sealed-off room. Upon activating the window shutters he comes face-to-face with Olivia Pierce for the first time. VEGA finds himself unable to stop Pierce, who appears to be rerouting the UAC's power to the "Argent Energy Tower", which Hayden deduces as Pierce trying to force open a portal to Hell with the tower as the epicenter. Pierce leaves and Hayden tells the Slayer that the system lockout she initiated will mean they must shut things down manually before she can gather enough Argent energy, which can be done by turning off the Argent energy induction filters. The Slayer passes through another airlock that takes him to the Argent Facility. Upon finding the first filter, Hayden mentions that he only wants to temporarily bring the tower offline to deal with Pierce and that the filters are to be treated gently. The Doom Slayer, considering the havoc Argent energy is introducing, instead opts to destructively crush the filter to ensure that humanity's use of Argent energy is stopped, much to Hayden's disdain.
Against Hayden's constant protests that Argent energy is vital to humanity, the Doom Slayer destroys the remaining three Argent energy filters but the problem remains that Olivia Pierce is still intent on achieving her goal so the only option remaining is to seek out and kill Pierce herself. VEGA says that he can track Pierce and that she is at the Argent Energy Tower. The Slayer crosses the precarious Argent energy pipeline to make contact at the tower with the help of some booster augmentations for his boots found in a nearby locker room. At the base of the tower, the Slayer once again sees Pierce, who is carrying a cylinder glowing with red energy. Hayden tells him that it's an Argent energy accumulator, which would give Pierce enough energy to unleash the Hell portal manually. The tower being offline doesn't break the bond between realities so with enough energy it is still possible to create a portal.
The Doom Slayer ascends the interior of the tower in a race against time to prevent Pierce from opening the Hell portal, which may be unstoppable should she succeed, fighting scores of demons along the way. However, upon reaching the pinnacle, the Slayer finds he is too late and Pierce activates the accumulator before jumping into the energy stream and vanishing. An endless cadre of demons pour out of the rupture but before the Slayer can deal with them, a countdown starts and then a white light bathes the entire area.
Hell on MarsThe Doom Slayer arrives in front of some hellish architecture while a demonic voice announces that "he is here". Passing through a doorway, the Slayer comes across a holographic echo of a recon team led by Dr. Hayden that shows the green-tinted light poles are wayfinders left behind by the expedition. Being inside the demons' domain changes nothing for the Slayer and he continues his bloody crusade through their world to find a way to return to the Mars UAC installation, which includes a first encounter with the mighty Baron of Hell. Through his trip across Hell, the Slayer encounters tablets that read off accounts of a previous ravage through Hell, describing in horrific detail the level of threat posed by the Slayer.
Following the wayfinders left behind by the UAC leads the Doom Slayer to the Mars portal. At the portal, the Slayer sees another holographic echo, this time of the moment when Hayden first came across the sarcophagus the Doom Slayer awoke in. The Slayer activates the portal and reappears back at the Argent Facility, now a smoldering ruin in the wake of the earlier demonic invasion. Hayden greets the Slayer by informing him that the situation is grim; with the Argent Tower destroyed the unrefined Argent energy flows freely into Mars through the open portal from a location in Hell referred to as "The Well". However, as the Slayer has managed to come back from the other side, Hayden believes there still yet lays hope and tells the Slayer to consult a VEGA terminal so he can lay out his plans.
Once at the terminal, Hayden explains that Olivia Pierce has been studying an artifact within her private laboratory; a fractured stone that bears the rune brushed on the Doom Slayer's helmet. Pierce's personal study, the "Lazarus facility", is in the Advanced Research Complex; a cluster of buildings that is approximately 80 kilometers from the Argent Facility. VEGA activates the UAC tram line to the Advanced Research Complex but mentions that a corporate clearance card will be necessary to actually board. The Slayer sets off to explore the ruined facility for an access card, which he eventually finds by following the hologram of a "Pinky" demon that carried off the remains of a UAC employee to the spaceport atop a tall building. Before leaving, another hologram appears showing Pierce and Hayden disagreeing upon what to do with Pierce's research. The Slayer takes his access card to the tram and boards for the Advanced Research Complex.
The Plan is EnactedAs the Doom Slayer rides the tram into the Advanced Research Complex, Dr. Hayden comes on the comms to invite the Slayer to their first face-to-face meeting in his office on the second floor. The Slayer fights his way through the complex until he arrives at the doorstep of Hayden's office, along the way witnessing another recorded argument between Hayden and Olivia Pierce, with Hayden expressing concerns for Pierce's growing insanity. Upon activating the shutter to enter Hayden's office, a massive jolt of energy brings the Slayer to his knees. The shutter opens to reveal the towering frame of Samuel Hayden's cybernetic body and Hayden explains that he installed a prototype "interdimensional tether" inside the Praetor suit that will allow him to pull the Slayer back from the other dimension at will; Hayden's use of a less-than-voluntary method being his doubt that the Slayer would have cooperated willingly.
Hayden invites the Doom Slayer to his office where he explains in depth about the stone he alluded to earlier. Referred to as "the Helix Stone", it is considered the UAC's most closely guarded asset, originally taken from Hell in the fledgling period of the UAC's Argent energy program and Olivia Pierce's personal fixation. Decoding the Helix Stone allowed the UAC to understand and harness the power of Argent energy as well as what allowed the UAC to uncover the Doom Slayer's sarcophagus. As the Stone was crucial to making Argent energy usable to humanity, Hayden believes it can now be used by the Slayer to stop the flow of The Well. Hayden directs the Slayer to the Lazarus Labs where Pierce has been keeping the Stone, which is held behind an access-guarded elevator elsewhere in the Advanced Research Complex. Hayden also makes mention of something else that may be found along the way that the Slayer might find enticing.
The Doom Slayer heads for the Lazarus Labs, first passing through the facility's "BFG Labs". After clearing out the demons in the lab, using a carcass to access the inner sanctum, and managing to avoid the deadly laser security system installed around it, the Slayer is able to claim the lab's crown jewel: the prototype weapon "BFG-9000" that Hayden had hinted at earlier. After descending below the labs, the Slayer witnesses a holographic recording of the evacuation of the BFG Labs above before proceeding to the Lazarus Labs elevator. Upon clearing out the demonic infestation surrounding the elevator, the Slayer descends into Olivia Pierce's personal laboratory. Hayden explains to the Slayer that he will be one of the very few who has ever seen the Helix Stone personally and that he may be able to use it in ways that they have yet to decipher.
Within the labs, a holographic recording plays that alludes to Pierce having knowledge of what the Doom Slayer may bring about if he reawakens. Further into the facility, Hayden explains the Lazarus Labs were the origination of the hellwave that first set the UAC into chaos, triggered by Pierce's inability to fight off demonic influences (which the Slayer gets to witness when a hologram near the Pierce's office shows him a conversation between Pierce and a demonic voice). Once at the Helix Stone, the Slayer deactivates its security matrix in order to gain access by visiting Pierce's secret office. The Stone sends a vision to the Slayer of a strange artifact that Hayden recognizes as "The Crucible" and realizes that was Pierce's goal all along. VEGA pinpoints the location of The Crucible to a uncharted region of Hell designated the "Titan's Realm". In order to travel there, another Argent accumulator will need to be used, which VEGA also gives coordinates for. Hayden explains that the accumulator will automatically send the Slayer to Hell once more after it is removed, though his tone of voice indicates that acquiring the accumulator will not necessarily be a simple task.
Into the Fire AgainUpon making contact with the Stone, a new wave of demons pour into the Lazarus Labs but the Doom Slayer stands tall and pushes through. After clearing the lab, a secret staircase surrounding the Helix Stone opens and leads the Slayer down a spiral into a strange church-like room that appears to have been used by Olivia Pierce to conduct a form of demon worship (which is reaffirmed by a holographic echo showing Pierce and her followers in the middle of an arcane ritual). A lift in a back room of the chapel allows the Slayer to exit and continue on to the accumulator. What lays beyond is the place where Pierce first unleashed the hellwave, which is depicted in yet another holographic recording. The Slayer shuts down the portal and then Dr. Hayden comes over the comms to explain why he needed the Doom Slayer originally; as insurance if and when Pierce became too unstable and too reckless.
The Doom Slayer moves past the hellwave origin point and through another demon-infested room before taking a circular lift to a large doorway. Hayden explains that he will need to pull the accumulator currently housed inside the "Cyberdemon" that lies beyond this door. Removing it will once more send the Slayer to Hell where he can locate The Crucible, at which point Hayden and VEGA will return the Slayer to Mars with the tether. Undeterred by what lies ahead, the Slayer opens the the door and is immediately yanked inside by the massive Cyberdemon. An epic struggle ensues but the Slayer is able to best the beast and pulls the Argent accumulator from its chest. However, when the accumulator destabilizes and pulls the Slayer back into Hell, the residual Argent energy within the realm also revives the Cyberdemon and the battle continues. The Slayer continues his assault on the Cyberdemon before severing one of its horns and using it to kill the monster once and for all by slicing off the top of its head.
Taking a teleporter within Hell brings the Doom Slayer to the remains of a massive demon called "the Titan". The Slayer fights through the hordes of lesser demons that have taken up residence inside the remains while also navigating the connected maze-like labyrinth. At the end, he locates another teleporter that warps him to another demonic temple. As the Slayer explores the complex, a demonic voice issues orders to the armies of Hell to impede the Slayer at all costs but it is futile. Finally, after extinguishing all the demons in his path and conquering the maze, the Slayer passes through a door that leads him to a glowing inscription on the wall. Upon touching it, the Slayer is transported to a circular arena where The Crucible sits inside a pedestal at the center. However it descends into the floor as a massive suit of armor emerges up. A strange worm-like creature crawls inside and together the pair form the "Hell Guard"; the protector of The Crucible.
The battle begins with the Hell Guard employing a magical barrier to deflect all offense but by taking advantage of brief moments of vulnerability, the Slayer overcomes and destroys the Guard with its own staff. But in its wake, two more Hell Guards emerge and continue the fight for their fallen comrade. Despite the uneven odds, the Slayer fends off both foes and rips out the last Guard's pilot, tearing the parasite in twain. Its last protectors defeated, The Crucible rises out of the center of the room and is claimed by the Doom Slayer, calling forth echoes of the "Night Sentinels", the order of holy knights with whom the Doom Slayer originally fought alongside in his war against the legions of Hell. Hayden's trans-dimensional tether activates and pulls the Slayer back to Mars with The Crucible.
The Final PushThe Doom Slayer reappears on Mars at an unknown location. Dr. Hayden tells him that they have a plan to shut off The Well and VEGA explains that the Slayer has been transported to site of VEGA's central processing core in a frigid region of Mars. VEGA requires massive amounts of power to operate but so much power can also be used to warp the Slayer to The Well. However, it comes with the knowledge that doing so will terminate VEGA. VEGA can only be overloaded at the source so the Slayer's first task is to bypass any security on the software level, which is done by shutting off the security network that protects the framework at a nearby maintenance facility. VEGA then recommends disabling the cooling system in order to speed the meltdown once it is initiated.
Within the cooling center, the Doom Slayer destroys the four cooling coils then eliminates the resident demons in order to break through the lockdown. The only thing remaining now is to travel by rail to VEGA's processing center to initiate the meltdown. Powering through yet another demonic horde the Slayer arrives at VEGA's central processing core as VEGA's A.I. begins to break down. Hayden informs the Slayer that once the meltdown is fully initiated, the resultant energy vacuum will pull the Slayer back into Hell to the preset coordinates of The Well, at which point the Doom Slayer will be able to close The Well using The Crucible and permanently cap the portal to Mars in the process. The Slayer goes about ejecting VEGA's neural processors but before he initiates the final phase of the meltdown, the Slayer takes a moment to back up VEGA's A.I. as Dr. Hayden offers his gratitude for VEGA's assistance and sacrifice. The core destruction is engaged and the Slayer is once again ripped into Hell.
The Doom Slayer arrives on a section of Hell referred to as "Argent D'Nur"; a separate realm that was completely consumed by the Hell dimension into its own and the source of all Argent energy. Relics of the Night Sentinels and the rune of the Helix Stone are seen throughout the mutated realm. The Slayer's final objective is to locate the bodies of "the Wraiths", the beings that originated in Argent D'Nur and are now being channeled by the demons of Hell for their Argent energy, and disconnect them from The Well. Within the Wraith Cathedral, the Slayer gets his first glimpse of The Well, which appears as a glowing red orb fueled by three points that pushes Argent energy into the sky.
The Well is closed by traveling to the three points and inserting The Crucible into the remains of the Wraiths, turning off that source of energy. Doing so also gives guidance by the echoes of the Night Sentinels, who point the Doom Slayer towards the next Wraith. The demons fight ferociously to prevent the Slayer from destroying their Argent energy resources but there is no stopping the Slayer's mission and he savagely pushes through the resistance.
With the final Wraith disconnected from The Well, the source is broken once and for all and The Well is no longer able to provide Argent energy. The sound of a haunting laugh is heard once the Doom Slayer disconnects the third Wraith. Following the sound, the Slayer arrives in a deep underground cavern with a demonic symbol embedded in the floor. There he finds Olivia Pierce, who has degenerated into a shambling, bloody slave of the demons. Seemingly regretting her decision to assist Hell in conquering her own dimension, she is suddenly overtaken by a surge of Argent energy and sucked into the floor. In her place, a four-legged cyborg monstrosity appears: the Spider Mastermind. The final confrontation of the forces of Hell begins with the Doom Slayer throwing everything he has to defeat the beast.
The fight is long and brutal but the Doom Slayer's persistence is unyielding and eventually he weakens the Spider Mastermind to the point where he is able to approach the creature and fire a point-blank BFG-9000 shot directly into its mouth, blowing off the top of its exposed brain and ending Olivia Pierce and Hell's inquisition on Mars once and for all. His mission complete, Hayden pulls the Slayer back to Mars one last time. Dr. Hayden greets the Slayer upon returning but the meeting is anything but cordial. Hayden relieves the Doom Slayer of The Crucible while recalling the sacrifices needed to come to this point. Despite all that has transpired, Hayden refuses to concede that what was wrought outweighs the energy needs of humanity and that he will continue the lost work by using The Crucible. Hayden doesn't kill the Doom Slayer but is not willing to let him continue to oppose the efforts to harness Argent energy either. Instead the tether system activates once more and pulls the Slayer to an unknown location of Hayden's choosing with Hayden's expectation that they may once again meet someday...