Back at last. After days of hard fighting in space, you have returned home on well-earned leave. You are one of Earth's crack soldiers, hard-bitten, tough, and heavily-armed. When the alien invasion struck Mars, you were the first on the scene. By killing, killing, and killing, you won. You stopped the invasion, saved Mars base, and became a war hero. What they don't talk about so much is that you were the only survivor.
But that's all behind you now. You have quit the military, and are heading home. Your drop pod lands with a crunch. You open them up, and look out. Damn! The city ahead is on fire. What the devil is going on? You stagger forward, clutching your side arm. Packs of refugees are fleeing the flaming metropolis. A band of them shriek in terror. You squint. What's that? Someone is attacking the refugees. You rush up and blast away, killing the troublemaker. It looks like a human, but something is wrong. His mouth is filled with half-chewed flesh, and he's all messed up, like a zombie from a bad horror movie. Hell, not again!
You can feel it. It's all starting again, just like on Mars. First, people are taken over, turned into cannibal Things. Then the real horror starts, the deformed monstrosities from Outside. But now it's on Earth.
It turns out that the invaders are all over. Monsters range from Tokyo to Timbuktu, from Stockholm to Scranton. Billions are dead. Some people have been transformed into flesh-eating mutants, but a few, a very few, are still alive and fully human. The wise men of humanity have evolved a plan to save what's left of the human race. They have built enormous ships to carry the remaining people into space, safely away from the ruined world.
Unfortunately, Earth's only ground spaceport has just been taken over by the demons. They've instituted a sort of force field -- a flame barrier -- over the port; no ships can land or take off. You gotta go back into action. The pathetic remnants of Earth's soldiers are making an assault on the invaders at the spaceport. If you win, you might be able to turn off the barrier, so that the ships can leave, and Homosapiens may not go extinct just yet. If you lose, that's it. Humanity is history.
You and your comrades attack. Soon, brave men drop like flies. You lose track of your friends, though sometimes you can hear them scream when they die, and the sounds of combat echo from deep within the starbase.
Something hisses with rage from the steel tunnels ahead. They know you're here. They have no pity, no mercy, take no quarter, and crave none. They're the perfect enemy, in a way. No one's left but you. You... and THEM.