Setting is late 1800's in New Mexico. Jake Lonergan (Daniel Craig) wakes up in the desert, injured and with no memory of who he is or how he got there, but with a metal bracelet firmly attached to his wrist. After defeating some fellows who assume he's a wanted man, he then gets his unusual wound tended by a kindly minister in a nearby town. He tangles with the spoiled troublemaking son, Percy, of the local cattle baron Colonel Dolarhyde (grizzled Harrison Ford). Percy accidentally shoots a deputy while bullying the locals and the sheriff (Keith Carradine) knows he has to be charged even though Percy loudly tells everyone that his Paw will get him out like he always does. The sheriff recognizes Jake as a wanted felon so Jake and Percy are chained together and on their way to Santa Fe when Dolarhyde and his men arrive to spring Percy. At that moment low-flying spaceships arrive and use metal lasso-type things to snatch various people and fly away (including Percy and the sheriff). Jake discovers that his metal gizmo can fire a huge powerful blast and is an effective weapon against aliens and their low-flying ships.
A posse of townsfolk head out to find the aliens, one of which has been on the ground and left tracks. A mysterious woman named Ella (Olivia Wilde) from the town keeps telling Jake that he must remember where he came from because he knows where the aliens are, but Jake still recalls nothing despite a few flashbacks.
The posse encounters Jake's old coach-robbing gang, who are still mad at Jake for taking some gold they had stolen and leaving them to settle down with a woman. Jake has flashbacks of a beautiful woman but they argue over some gold he has which she calls "blood money"; the gold actually drew the aliens to their cabin and they both had been snatched.
A small tribe of Indians surround the dwindling posse after Ella dies from a wound inflicted by an alien. The men are taken captive and Ella's body is tossed on the campfire. The 'woman' Ella was actually a different alien whose planet was destroyed by these interplanetary thugs who torture others to discover their weakness. She resurrects out of the fire in a column of light and takes the same form again. This impresses the Indians and the Chief gives Jake a potion which helps him regain his memory; he knows where the aliens are and that he got the bracelet in a fight while escaping them.
Jake returns to the robber gang also persuades them to join the posse.
They all arrive at the mothership which is planted mostly underground in a desert canyon cliffside. Dolarhyde, the Chief and the collective posse engage the aliens with pistols, spears and arrows while Jake & Ella go inside, find and free some of the people who were snatched. Ella kisses Jake to help him "not think" so that the bracelet falls off; she goes deeper into the core of the spaceship using the bracelet to destroy it (and herself). Jake escapes and rejoins the townsfolk and their rescued loved ones and they all watch the ship explode. Percy and the sheriff are among the rescued.
The town begins to return to normal and is booming since the discovery of gold nearby. Dolarhyde offers Jake a job and the sheriff is willing to swear that Jake Lonergan died in the battle, but Jake rides off into the sunset.