On July 2, a giant alien mothership enters orbit around Earth and deploys several dozen saucer-shaped 'destroyer' spacecraft that quickly lay waste to major cities around the planet. On July 3, the United States conducts a coordinated counterattack that fails. On July 4, a plan is devised to gain access to the interior of the alien mothership in space, in order to plant a nuclear missile.
Canaan, a mysterious gunfighter left nearly blind from Civil War combat, roams through Mexico with a baby he has sworn to protect. On his way to a town where a family will supposedly adopt the baby, Canaan passes through a border town where U.S. Cavalry officers assigned to deliver a shipment of silver are under attack from bandits. With some reluctance, Canaan steps in to help the soldiers.