The film opens with Wild Bill riding onto an apparent battlefield. An elderly Mexican man steps forward and asks in Spanish, > Qué pasa aqui? The outlaw answers with a taunting line, La Serpienta Del Diablo. He then approaches a carriage, a woman tumbles out, a copperhead uncurls from the shadows, and [Wild Bill] shoots it dead before riding off into the dust.
The next day in a sunbaked town in New Mexico, the outlaw stalks into a dusty saloon to order a drink and to ask for [Murphy]. The bartender, Garrett, delivers the grim news that Murphy is already dead. Before long, a swaggering gunfighter named Jesse Evans and his crew—Will, Roscoe, and Ponce Domingues—march in and trap Bill at gunpoint, insisting that he listen to their terms. Bill warns that a swarm of highly aggressive snakes is headed straight for the town, but Jesse scoffs and demands a duel. A chaotic shootout erupts: Jesse is struck in the chest, Bill is wounded in the arm, and the saloon erupts in a hail of bullets. The attackers are momentarily repelled when Garrett, the town’s doctor Josiah (and a sturdy blacksmith named Tannen) and others step in with weapons drawn, forcing Jesse to retreat and ride off toward Lincoln.
Josiah patches Bill’s arm while they survey the damage: the horses and Henry the stable worker have fallen to the serpentine attack. The snakes surge into the saloon, leaving a trail of chaos and body blows, and the town’s defenders realize the danger is far from over. A lookout from the window reveals the first mountain-outbreak of a vast swarm, and Bill’s horse collapses in the street from the assault. Amid the panic, Bill urges Sheriff Mercer to arm every gun in the office. Will, his girlfriend Jane, and the saloon’s Darla shepherd the women and children into the bank’s safe room for protection. Tannen leads them to his Blacksmith shop, where he unveils a wartime arsenal: a Gatling gun and a flamethrower, leftovers from old battles. Working together, they create a water-and-barrier pool to corral the growing threat and prepare for a defense against the creeping flood of snakes.
That night, the threat arrives in force. A colossal swarm of copperheads pounds the town, and the defenders are forced back into Murphy’s hotel room. In a grim moment, [Ponce] is slain by snakes at the window as the others realize Roscoe is somehow the traitor behind Murphy’s murder. They trap Roscoe behind a door and block the snakes’ approach, but the wall between rooms proves too thin; the serpents tear through it, forcing the survivors to retreat into Murphy’s room.
Darla is killed as the battle spills into the corridor, and the group discovers a hidden passage that leads to the hotel’s back wall. They break through and realize the only way out is by dynamite from a shed in the alley. Will and Garrett sprint to fetch the explosive while Jane and Josiah keep watch. A snake slithers into the room, but they drive it back with quick gunplay and a volley of shots. Bill hands Will his gun, and Will detonates a charge that cripples the swarm—but not before the hotel and its occupants endure a brutal, claustrophobic struggle for survival.
With dawn breaking, the survivors regroup and tally the cost: the snakes appear to be babies, a sobering discovery from the careful eyes of [Josiah]. Roscoe somehow survives the explosion and returns, only to be consumed in a nightmarish moment when a 40-foot mother copperhead erupts from the ground and devours him. The remaining fighters sprint toward the sheriff’s office, seeking cover as Mercer orders a city-wide stand. Mercer tries to seize the moment, ordering dynamic actions that include tying a dynamite fuse to Jane as a decoy, but Bill punches him and locks him away.
A bold plan forms: they lure the bulk of the snake into the Blacksmith’s workshop, fashion a chain, and use fireplace pokers as improvised spears to drive a Gatling gun. Mercer, now freed by a hungry paranoia, rushes to grab the keys, but the enormous snake surfaces again, chasing Will through the room. Bill hurls the chain over the creature’s neck, while Garrett fires the pokers into its thick neck and Jane blasts the beast with a shotgun blast to the side. The monster retaliates, nearly swallowing Jane, and even destroys the Gatling gun with a brutal head strike. Bill recovers the shotgun, delivers a fatal pair of shots—one in the back, one in the mouth—and finally slays the colossal menace. Mercer erupts in the struggle, dropped under the weight of the serpent and the fallout of his own pride.
As the Pinkertons arrive and search for Bill, Garrett reveals the truth—that Bill is Murphy and that a little-known stash of the missing man’s gold is hidden somewhere nearby. Bill urges the Pinkertons to rescue the women and children hiding in the bank’s vault, then makes a rapid exit with Will, Garrett, Jane, and Josiah on their horses. They ride westward to Lincoln while the Pinkertons pursue. The cost of victory weighs heavily on Ms. Murphy, who finds her father’s gold watch in the aftermath, and, in a quiet, symbolic moment, she looks up to see Bill framed against a blazing sunset as he rides into the horizon, a lone rider who has survived but not forgotten to carry what he’s done.
