In 1873, in Arizona, an amnesiac outlaw (Daniel Craig) finds himself face to face with some bounty hunters (they have Indian scalps dangling from their saddles). He guns them down with this bracelet weapon he has locked on his arm. Then, he goes into the nearby town and people recognize him as wanted man Jake Longergan. He had stolen some gold from a rich rancher, Woodrow Dolarhyde (Harrison Ford) but what happened after that is a blank for him. The first forty-five minutes sets up the whole Western scenario, introducing such people as Dolarhyd's sleazy son, Doc (Sam Rockwell), the sheriff (Keith Carradine) and Ella Swenson (Olivia Wilde), who may know what happened to Lonergan. So, when the aliens appear it's almost as an afterthought.
The aliens are after the same thing humans in the West are after--gold, and have this huge mining ship in the mountains. They look similar to the aliens in both SUPER 8 and FALLING SKIES, which makes sense since they are all produced by Steven Spielberg.
It was sort of amazed at how boring this movie was. I think it's a case of "too many cooks in the kitchen" as there are eight writers involved. COWBOYS & ALIENS is perhaps the most disappointing movie of the Summer of 2011...