In 1880, several townspeople have gathered outside of town to hang Joe Caswell, a cowboy who shot a man in the back. The reverend offers him a chance to pray, but Caswell tells them to get on with it. The judge says that Caswell is an evil man but offers him the chance to say a few last words. Caswell tells his victim's father that he would have done the same thing again, and he defended himself. The old man points out that Caswell shot his son in the back, and hopes that Caswell will die long and slow. Caswell tells them to get on it with and the judge's man slaps the horse forward. Caswell dangles from the noose... and then disappears.
In 1959, Professor George Manion wakes up Caswell. He tells the cowboy that he is now in New York City in the year 1959, and shows him a time machine that he's built. Manion informs Caswell that he has distinguished future as the first man to travel through time. When Caswell clutches at his throat, Manion sees the rope marks on his neck.
Later, Manion transcribes his notes on Caswell. According to Caswell, the cowboy was riding herd in Montana in 1880 when he found himself in the laboratory. Manion isn't convinced, and worries that he has brought a murderous primitive from the 19th century to the 20th.
Caswell comes to see Manion and asks for a cigarette. The professor tells Caswell that there are things in the new world of 1959 that he can't imagine, and Caswell demands to see them. When Manion opens the drape, Caswell backs away in horror, clutching at his ears. They discuss good and evil, and Manion tells Caswell that it hasn't changed. Caswell says that a deputy sheriff tried to beat the difference into him, and Manion talks about justice. He accuses Caswell of being a killer, and Caswell admits that he's killed dozens of people. Manion insists that he has to send Caswell back to meet his fate, and Caswell says that he has no idea what he's talking about. The cowboy attacks Manion as the professor goes for a hidden revolver, and Caswell smashes in his skull with a lamp. As Caswell turns to leave, he knocks over Manion's dictation machine. The recorded words of his suspicions about Caswell fill the room. Stunned at the sound of a dead man's voice, Caswell runs outside.
On the streets, Caswell shoves through the crowds and tries to escape the lights and sounds. He inadvertently ends a telephone booth and finds himself trapped, and smashes through the door. Caswell flees into a bar and smashes the jukebox when it play music. He turns to the bartender, setting Manion's revolver on the counter, and asks where the musicians are. Caswell demands whiskey and the bartender reluctantly hands it to him, assuming the cowboy is drunk or crazy. When Caswell sees a TV on the wall, he assumes that it's a window and the bartender demonstrates by turning it on. The TV is showing a cowboy program, and Caswell shoots the on-screen cowboy when he calls an off-screen opponent out. The bartender yells for the police as Caswell runs out. He shoots at a police car and then stumbles off, inadvertently dropping the revolver.
Caswell finally goes back to the laboratory. He begs Manion for help. A thief, Paul Johnson, comes into the lab and draws a gun on Caswell. He figured that Manion would be in bed and came there to steal the professor's money. Johnson assumes that Caswell is a thief as well and killed Manion. They fight for the gun and Johnson finally strangles Caswell with a window blind cord.
Once his competition is eliminated, Johnson tries to find Manion's hidden money. He inadvertently turns on the controls and then enters the time machine. The door closes behind him and he pounds on it, begging someone to release him, as lights flash.
In 1880, the execution party looks up at the empty noose... and John appears there, taking Caswell's place. They cut him down, but too late to save his life. As they take the body away, the judge and the reverend wonder if they executed an innocent man, unaware that cosmic justice has been served.