In 1880, cowboy Joe Caswell is ridden out of town and prepared for execution by hanging from a tree. When the reverend prepares to say a prayer, Caswell says that he doesn’t believe in prayer and tells the judge to make it short and get the hanging over with. When the judge gives him a chance to say any last words, Caswell says he would have done the same thing again. His victim’s father says that Caswell shot him in the back and hopes that Caswell will take a while to die. Caswell, unrepentant, tells them to get on with it. The judge says he’s an evil man and has one of the cowboys prod Caswell’s horse forward. The killer’s horse runs forward, leaving Caswell dangling from the noose... and then Caswell disappears, leaving an empty noose.
In 1959, Caswell wakes up. A professor, George Manion, tells him that he’s a long way from home. He explains that Caswell is in New York City in 1959, and shows him a time machine that he’s created. Manion isn’t interested in Caswell’s past, but tells him he has a distinguished future as the first man to travel through time. When Caswell clutches at his throat, Manion sees rope marks on the skin.
Later, Manion transcribes the information he’s gathered from Caswell. The cowboy has claimed that he was riding herd when he suddenly disappeared and found himself in 1959. Manion admits that he doesn’t like Caswell and is concerned that he’s taken a murderous primitive from another time and placed him in the 20th century.
Caswell enters the lab nervously examines the time machine, and then asks Manion for a cigarette. He asks for a look at his new world, and Manion says there are things out there that Caswell can’t imagine. He opens the window to show Caswell a New York City street. The cowboy clutches at his ears in shock, and Manion says that good and evil haven’t changed. Caswell says that he knows all about right and wrong from a deputy sheriff who beat the difference into him. Manion asks him about justice, and figures that he saved Caswell from an execution. Caswell boasts that he’s killed a whole territory worth of people, and Manion says that he has to send Caswell back to where he was at the moment of execution. Caswell says that Manion knows nothing about doing what it takes to survive. He punches Manion and throws him over a desk, and Manion goes for a gun. Caswell smashes a lamp over his head and takes his revolver. He activates Manion’s dictation machine, playing his assessment of Caswell. Caswell retreats from the dead man’s voice in horror and runs outside.
Caswell runs through the streets, shoving his way through the crowds as the lights and sounds overwhelm him. When a phone rings, he enters the phone booth to answer it and inadvertently traps himself. Fighting like a caged animal, Caswell smashes through the glass and makes his escape. Finally he goes into a bar and stares in shock at the jukebox. He pounds on it and finally breaks it with a chair. The bartender warns that he’ll have to pay for the damages, and Caswell takes out Manion’s gun and places it on the bar. He demands a bottle of whiskey and then asks what the jukebox is and where the music comes from. Caswell rambles about horseless carriages, and the bartender tells him to get some sleep. Caswell looks up at the television and mistakes it for a window, and the bartender turns it on. On the channel, a TV cowboy approaches the camera and says to draw. When he goes for his gun, Caswell shoots back and destroys the TV. The bartender yells for the police and Caswell runs out into the street. He fires at a police car, stumbles, away, and loses his gun.
The killer finds his way back to the laboratory and begs the dead Manion for help. A thief, Johnson, comes in and holds Caswell at gunpoint. When he sees Manion’s corpse, he figures that Caswell is also a thief. When Johnson threatens him, Caswell grabs the man and disarms him. They struggle for the gun and Johnson finally manages to strangle Caswell with a window blind cord.
Once Caswell is dead, Johnson searches for Manion’s money. He inadvertently activates the time machine’s controls and then enters the time machine booth. The door closes, trapping him inside, and there’s flashes of light.
In 1880, the execution party stares in shock as John’s body appears in the noose meant for Caswell. By the time they cut him down, it’s too late. They have no idea who he is, and wonder if they executed an innocent man.