Here's my horrible attempt at a story. :P
In the year 2168, the gigantic multi-planetary, multi-industry monopoly corporation known as the AGM have had near total control of the earth. True, the government is still there, but they are powerless to the will of the AGM.
The AGM was started in 2031, as some small science related thing. They eventually grew so much, that they controlled almost ever industry you can think of.
The AGM is working on "Project DarkGate", a top secret experiment for creating portal technology that manipulates the quantum states and somehow involves antimatter or something crazy like that. Research is going well so far, except...
There is an alternate universe, known as Hades. It is a world of darkness and despair, everyone for themselves, and survival of the fittest. Many alien races are there (the different kinds of monsters).
They invade other universes and conquer them all the time. At the heart of the most powerful of several militias in Hades, they discover a signal on the quantum radar (whatever the fuck that is), finding that there was direct and unencrypted manipulation of dimensions, they figure it must be part of a teleport system.
They do some more investigation, and decide that they want to invade our universe to steal our technology and enslave us.
The scientists notice unusual behavior on the monitors, it appears that the quantum shit is being directly manipulated by a 3rd party. They assume that it could be some friendly aliens that would like to meet us, so they decide to allow it to continue, so that mankind may finally meet extra-terrestrial life...
Turns out it didn't work too well. Swarms and swarms of alien monsters come in, kill everybody, and all kinds of things.
Now you have to fight off the alien invasion, as you're one of the few surviving security guards from the initial wave of attackers.