The Discovery on LV-426
By the 25th century, mankind had outgrown the bonds of Earth's gravity field. Numerous worlds had been terraformed and colonized. But in all its spacefaring career, mankind had not yet encountered any significant alien lifeforms, with one little-known exception.
On planetoid LV-426, a six-man mining crew had discovered the eggs of a particularly dangerous parasitic creature. All records of that discovery have mysteriously been misplaced and the six-man crew is listed as missing.
The Infestation of Vega 4
Now it is 2493. The colonists of New Shanghai, on the 4th planet orbiting the star Vega, in the constellation Cygnus, had absolutely no knowledge of the Aliens when they found an underground cavern. In the cavern was the ancient ruin of a crashed spaceship, buried in an ancient Vegaquake, and, in the spaceship, some unusual eggs.
The colonists thought it the most important and exciting scientific discovery of mankind's history. But then the eggs hatched.
The Nature of the Aliens
The Aliens propagate by planting their seeds in the bodies of living and unwilling spacefaring hosts. In this way the Aliens infest planets far away from their own. They have no scientific or technological ability.
Their society is that of a hive. Their sole purpose in life is to live and multiply at the expense of other creatures.
A Call for Help
After the unearthing of the eggs, the Aliens quickly infested the city of New Shanghai, and the Alien plague began to spread to other areas of Vega 4. The once-prosperous colony world seemed doomed.
The governing authorities sent an emergency distress call to Earth. As fate would have it, the message was received and understood by a nearby spacecraft bearing, by chance, real hope for Vega 4's salvation. Salvation, however, was the farthest thing from the minds of the spacecraft's passengers.
The Arrival of the Predators
Mankind was about to meet yet another alien race. This new race possessed intelligence and an advanced technology. This new race had a complex society based on Honor, Glory, War, and the Hunt.
Man was about to meet Predator. The Predators, however, were far more interested in hunting the Aliens. At long last they had finally found a creature which would provide truly challenging sport. Mankind would benefit from the fateful meeting of these awesome creatures.
The Ultimate Goal
If you are successful, you will clear both planets of all Alien creatures, including the Queen herself. If you picked up many skulls, you will have amassed honor for yourself in the eyes of your Predator clan.
But don't mourn for the Aliens. With their vile method of keeping their species alive, there may yet be more of them somewhere in the Galaxy. When they are found, there will be more opportunities to hunt this lethal creature.
No matter how much the humans of Vega 4 may owe the Predators for showing up at this crucial moment, the two races will never be friends. It's just not in the Predators' nature.