In the halls of the Deimos science station, the Marine makes his way through corridors of former humans, coming upon a recreation area. The Marine steps over the shadowed outline of a UAC logo, cast from a shaped window above. As he cleared out the zombies, he passes alcoves holding pots of various plants, comfortable chairs and modern art, detailed globes of Earth, Mars, Deimos and Phobos were placed around the room, the view of Hell outside could be seen from the distant windows. The bodies of slain UAC personnel were everywhere, the distant echoes of demonic activity continued to follow him. After checking another station map, he continued into the main corridor beyond, then into an intersecting path leading to a maintenance sector.

He walks up a short staircase and a gas pipe along the left wall suddenly bursts, shooting flames into the narrow passage, blinding the Marine for a moment. His vision fades back into focus on a Mancubus demon wading around a corner beyond the shooting flames, its excessive bulk taking up much of the hall's narrow clearance. The demon growls a gaseous tone and raises its arm cannons from the shadows on the Marine. He leaps back and lands at the base of the stairs as flames roasts the sloped ceiling just above him, heat radiating through the narrow hall. The Marine hastily reaches for the BFG 9000 and fires a quick low-charged shot, the prototype weapon shook violently in his hands. An unstable bio-force round shoots from the barrel, the surface fluctuating with waves of excess energy, electrical discharge escaped the compromised energy core. The bio-force round expands more and more until colliding against the sides of the corridor in front of the Mancubus as it detonates. The miss-fired round was not as effective but still managed to maim the demon.

The Marine looks down at the overheating weapon, he powers it off and examines it carefully. He finds a module behind a conduit that could be causing the problem. With the thought of not having the BFG, he suddenly feels extremely vulnerable, Echo Labs is bound to be swarming with demons. He remembers seeing the remains of a technician in the maintenance area along the way he came. He walks back and finds the body, looks it over and finds a toolkit clipped to their belt. He returns to the BFG with the tools and tries to repair it. He carefully removes the conduit blocking the problem underneath, revealing a glowing green module, it appeared to be out of alignment with a cylindrical array of plasma emitters. The tools he had weren't meant for this kind of job, but he still managed to carefully ease the green module back in place while sweat beads upon his forehead, overhearing the activity of nearby demons passing the connecting passages. He replaces the conduit and fastens it back into place, then powering the weapon up again. To his relief, the BFG 9000 appeared to be back in working condition. He puts away his prized weapon and continued on the path, arriving at a passage with a wide window stretching along the left wall. Glancing outside, he sees the red sky and the newly exposed surface of Hell far below Deimos. As he studies the alien landscape, a large demon vaguely similar to a massive larva passes from below the frame of the window, carrying a giant crimson jewel in its four arms.

The Mother Demon floats to the roof of the science station and glides toward a gathering of former humans that had amassed a pile of mortal remains. They were formed into a demonic growth on the station roof. The demon approaches the gore nest and places the jewel onto the crescent of twitching flesh, strands of tissue wrap around the gem, securing it in place. The creature drifts away from the growth, descending back down to Hell.

It passes through a dense green fog, then around pointed, jagged rocks. Lava spills from a distant cliff, flames shooting from the base of the falls. It floats to the sandy, black surface that was strewn with skeletal remains and roamed by Imp and Pinky demons, straying into a narrow cavern leading beneath. Descending deeper, it moves along stacked layers of bedrock leading to an area swarming with floating boulders orbiting at considerable speeds. Moving around the spinning rocks and further below, passing a morphing wall of drifting material and through a tunnelled opening covered by flesh and encircled with spikes. The creature drifts down the thorny passage and into a vast flesh-covered region.

Mountainous gore-nests tower above the deformed landscape, massive shimmering portals above the ribbed peaks were orbited by rocky debris. Descending further, the demon intersects a swarm of Cacodemon, swoops around gigantic flailing tentacles, then drifting past Hell Knights occupying the spiked, flesh covered ground. The Mother Demon enters into a fanged orifice protruding from the disfigured surface, the spikes receding as it approached. The flesh covered walls became more moist and slimy the further down it descends before coming to sections dripping with blood toward the far end of the passage.

The opening leads to a cramped area congested with crisscrossing strands of bloody tissue. The demon weaves around the teeming, confined territory, passing small spider demon, floating skulls and the remains of mortals tangled in the strands of gore, coming to a clearing at the edge of a cliff and toward a moving river of blood in the tangled valley beneath, passing screeching winged demon before floating over a blood-falls and finally arriving at an important and protected level of Hell.

Walls of ribbed flesh rose high throughout an area teeming with Arch-vile, Pain Elemental and Barons of Hell. A giant ancient skeleton dominates the valley, its skull's horns and ribs reaching upwards. Heaps of tortured writhing mortals moan beside a stream of blood as the demon nourish themselves on their suffering. The Mother Demon passes Barons feasting on mortals, throwing blood and parts in all directions as their fanged teeth rip their torsos. It floats above the imposing walls of flesh, beyond a field of impaled corpses and toward a growth-covered emptiness roamed by howling packs of Hell Hounds. In the distance stood a tall dark tower, once part of an alien building before it's world was consumed by the forces of Hell.

The Mother Demon drifts along the massive stone bricks to the base where a congregation of Dark Priests stood, making mortal sacrificial offerings to a motionless demonic god. The deity wasn't sleeping but also not entirely awake, its consciousness spread over the countless ages of Hell's past and future. On especially rare occasions, the Icon of Sin is witnessed consuming one of the regular offerings made by its loyal servant priests. The four hundred million year old demon stood looming high above within the narrow crevice of the dark tower, the air around it vibrates with disembodied, unintelligible whispering. It had rough armoured scales, its giant hands were tipped with retractable claws, rigid spikes were along its elbows and shoulders. The Mother Demon drifts up to the massive horned head and speaks in a whining, tortured voice.

It hovers before the giant deity for some time before its dark eyes began to slowly open, then moving for the first time in many years. The Icon extends its arm and the digits on its clawed hand, revealing a skull-shaped crystal with a red hue. The small demon drifts to the Icon's hand and takes the key as the Icon returned to its semi-dormant state. The Mother Demon moves to a growth of gore at the base of the tower, places the skull inside and it glows brightly.

Another fanged orifice parts, crimson energy brims up from the core of Hell to the flesh covered surface, a gore nest slowly emerges and spawns an orb of red energy, a glowing ray suddenly shoots upwards. The beam of demonic energy streaks from Hell's lowest depths through the many layers above, sizable chunks of the surface float out of the ray's path as it sailed toward Deimos drifting above and into the jewel held by the growth on the station's roof, sending the ray straight up as a large portal then opens in the sky, leading to the place of their next invasion. Hundreds of smaller portals materialize throughout Hell, the gateway on Deimos is connected to the network and cybernetic demon join the invasion force.

It was night-time on the Martian surface, some distance from Mars City was a forgotten, decommissioned UAC facility, the road leading to the building was strewn with rubble. Within the abandoned Delta Labs complex, the dormant classified portal systems in the dusty teleportation lab began to energize. Pricks of light from hardware indicators shone through the darkness as occasional system beeps pierce the silence. Flat, obsolete monitors power on and lines of code scroll down the screens before displaying system errors. A rising tone vibrates from the teleportation coils, a whirl of plasma began to envelop the gate and a portal flares to life. Demons from across Hell's domain began spilling from the gateway, infiltrating the base, crawling through air-vents and vacated hardware compartments. Mancubus and Hell Baron were soon forcing open sealed doors connecting to the abandoned main corridor, the decommissioned facility was completely over-run by demon. The creatures soon found paths leading to the surface of Mars, straying into the dark landscape.

Not far from the over-run facility was the excavation of an ancient alien settlement, archaeologists were engrossed in their work, writing reports, sifting through dust and examining artifacts under the illumination provided by floodlights on yellow tripods set around the site. One at a time, they pause from their work, noticing the ongoing noise echoing from beyond their site. Using binoculars, they scan the landscape for the source of the odd sounds which seemed to be getting closer. They spot the form of alien creatures advancing toward their excavation, the lead archaeologist orders the team into the rover truck, leaving almost all their equipment behind. The hoard was a couple hundred meters away when the driver guns the engine, throwing a cloud of dust into the air as the truck roars across the gravel road. They use the radio to report the sighting to Mars City as they speed back to base.

Within Mars City, the colony security chief immediately activates the distress relay signal upon receiving the report, then began issuing orders to the Marines and unlocking cabinets of plasma cells throughout the command center. On the bridge of the UAAF Vapor, the communications officer reports the distress signal to the bridge commander.

Captain Revok was in his personal quarters with Dr. Carmichael, lying in bed, sweat dripping from their bodies after their dinner together turned spicy. A call from the bridge fills the cabin as they were lying beneath the sheets. Annoyed, Revok climbs to his feet, transfers the call to his mobile and walks into the corridor.

"This better be important" Revok says.

"I'm sorry, sir. A distress signal was picked up from Mars City, there's been sightings of the aliens outside the base" the commander explains.

"Understood. I'll be on the bridge soon, Revok out" he says, hanging up the call and stepping back inside his cabin.

"Is something wrong?" Susan asks, concern drawn upon her face.

Revok hesitates as he picks out a fresh uniform from the closet.

"Mars City has reported demons sighted on the surface" Revok explains as he changed clothes.

Susan looks down in distress.

"No, not again" she laments.

Revok's head was spinning with a dozen competing thoughts.

"Susan, take the shuttle and get back to Earth. The situation is too dangerous" Revok instructs.

"What are you going to do, Michael?" Susan asks.

Revok left the question unanswered. He slips into a pair of boots, moves out of the room and enters the bridge only a few steps away.

"Let me see that transmission" Revok orders after walking through the door.

The communications officer steps aside to allow the captain to see her holo-monitor. He carefully reviews the distress call, then turning to face his bridge commander.

"I'm going down there, you're in charge of this operation now" Revok states.

The commander took on a look of shock, mind reeling by the weight of responsibilities coming under his command.

"What?! You can't abandon the mission!" the commander snaps.

"My daughter is in Mars City, I'm going to protect her" Revok explains.

"Oh, I had no idea. Very well, sir" the new Captain concedes.

"Don't worry, just follow the orders. You'll do fine" Revok reassures.

"I won't let you down" the Captain returns.

"Bring the ship into range of the planet for an atmospheric drop" Revok instructs.

"Helm, carry out the order" the Captain says.

Revok walks off the bridge and moves to a nearby gun cabinet, taking a plasma rifle and filling a bag with cells and grenades. He makes his way down to the spacious hangar bay and grabs an equipment mover and pushes it to a drop pod module against the far wall. Revok loads the drop pod and pushes the mover over to the air-lock door. He positions the module against the barrier, along coloured markings on the floor and locks the module's braces into place. Revok opens the drop pod, placing his rifle and bag, then climbing inside before it seals shut. Revok uses his own authorization to cycle the chamber, open the air-lock and drop the pod. The airlock slowly opens, the red planet looming beyond. The module unlocks the pod, releasing it into the atmosphere, falling faster and faster through the condensed gases and plummeting to the Martian surface below.