Brody, Grace and 6 others are seemingly dropped in this forest. They're experienced fighters or killers (either legally in the army or as doctors who do research, or illegally as convicts), but have no recollection of how they got there. Almost as soon as they arrive, they come under attack, and decide to join forces to save themselves.
As the story progresses, they find out that they "aren't in Kansas any more" and are being hunted by the Predators that we saw in the original Arnold Schwarzenegger Predator movie. They decide to kill their hunters (it turns out they're on a game preserve), and steal their ship to get off the planet. The rest of the film follows their attempts to achieve this feat.
In theory this shouldn't be that bad. It's not a bad set up. The problem is that Adrian Brody's no Schwarzenegger and Topher Grace is no Jesse Ventura. I mean Grace (who plays the doctor with a knowledge of how the government uses poisons) spends most of the film running round the jungle screaming like a teenage girl, which makes you wonder why he was there in the first place.
All in all, not a great film.