After the events of AVPR, a meteor hits the moon and forms a new planet. Weyland-Yutani researches the planet and find that the metal in the meteor is also in the Predator weapons. The predators need this planet for the ultimate ritual, alien and human hunting. To ready the planet the predators send 2 preds to the planet to set up 11 alien eggs and humans so they can hunt.
Weyland-Yutani continues their research and find that the bio weapons orgamism is also on the planet, the alien. With that in mind, Weyland-Yutani sends 14 people to the planet to capture the creature. The ones sent to the planet are unaware of what's going on, they were told that there's an unknown creature on the planet and they must either destroy it if it's hostile or capture it if it's not. This is why they need the USCM.
To ensure the capture of the alien, Weyland-Yutani gives specific orders to an andriod to stop at nothing to bring back the alien, the crew is also unawre of this. They leave and on their way there, six predators are sent to the planet for the hunt. Entering the atmosphere of the planet the ship crahses but not to the point where they can't leave, they turn on the back up genterators and go about their business.
They continue with the plans and explore the planet and leave three others to watch the ship. While out they find nothing more than a waterfall. They all come back to the ship and discuss what they found and what they didn't. Then the back up power goes out.
From then on it's chaos and a fight to stay alive.