It's set in West Texas and sees Brolin stumble on the aftermath of a shoot out between an Hispanic drugs gang and their American buyers in which both sides kill each other. Having no need for the drugs, he takes the money and (he hopes) disappears. Unfortunately, both sides in the disagreement work out who he is and he goes on the run. The story then focuses on Brolin, the police and the hit-man who is sent to kill him for taking the money. Harrelson is paid to track down the hit man by the American drug cartel as back up to the hit-man.
It's a really good film. I'd recommend everyone go and see it, but be warmed, the ending sucks majorly. We the ending better, this'd be my favorite film of the year.