From the developer's website:
The endless darkness and the somber, long passages of a military complex have become the abode of evil, as thousands of blood-thirsty creatures fill its offices, storehouses and mysterious laboratories.
Your mission is simple: clear the base at all costs. You will be provided with explosives to help you gain access to the teleports from where thousands of pitiless creatures pour. A stationary gun will aid in the defense of the area. You have been granted access to the most advanced weapons technology money can buy. As you earn your pay, you can equip yourself with additional weapons in the arming area and biomechanical implants that will make your fighting abilities super human.
The alien invasion has begun, we have one chance, and that is to stop them cold in their staging area. Do not allow them escape this facility, you are our last hope. The fate of humanity now depends on you!