Suave, witty, and charming Roger Thornhill is an advertising executive who seems perfectly happy with his breezy and boozy New York life. He's about to take a two-martini lunch with business colleagues and then go to dinner and the theater with his mum. But almost immediately after the opening of the film, he's mistaken for a man named George Kaplan…and his life suddenly turns upside down.
He's abducted by two men working for a man who claims to be a Lester Townsend. This meanie—who turns out not to be Townsend but instead one Phillip Vandamm—promptly tries to force Thornhill, mistaken for Kaplan, to come clean. It's not totally clear who Kaplan is at this point, but we do know that he's some sort of spy.
Thornhill refuses to come clean—naturally, since he's not really Kaplan. But nobody at the Townsend home believes him, so they try to kill him by pouring a bottle of bourbon down his throat and forcing him to drive drunk all over steep and scary roads. (Yeah, it was the 50s.) Thornhill barely manages to survive this drunk-driving episode but is picked up by the police.
No one believes Thornhill's story. When he takes the police to the Townsend home, they meet "Mrs. Townsend," who tells the police a made-up version of the story to cover up the conspirators' actions on the previous night.
Thornhill decides to take matters into his own hands. (Would you expect any less from a dude who looks like that?)
First he visits the Plaza Hotel, where he's learned George Kaplan is staying. But George Kaplan's not there; no one at the hotel has really ever seen the guy.
While at the Townsends', Thornhill learned that Lester Townsend would be addressing the United Nations that afternoon, so he heads to the UN. He finds Townsend, but this Townsend's not the man he spoke to the other night. No sooner does he begin to put two and two together than Townsend keels over by a knife thrown by one of the men who'd kidnapped Thornhill the day before. Holding the knife he pulled out of the man's back, Thornhill looks to be the killer.
Cut to an unnamed spy agency, where we learn that Thornhill's wanted for the murder of Lester Townsend. (Surprise!) A motley crew of agents realize that Thornhill's been mistaken for Kaplan and that Townsend's been killed as a consequence of the mix-up. The team considers their options and decides to do nothing. Any course of action to protect Thornhill would risk putting their main undercover agent (alias Number One) in danger.
Hearing that Kaplan was next headed for Chicago, Thornhill jumps on the 20th Century Limited train headed west. Once aboard, he meets Eve Kendall, a seductive blonde who repeatedly helps him evade capture by the police. Why's she being so nice? That turns out to be the million-dollar question. Then we see Eve slip a note to none other than… the creepy guy who pretended to be Mr. Townsend and had Thornhill kidnapped.
It's all coming together…
After their arrival in Chicago, Eve sends Thornhill to the middle of nowhere under the pretense of meeting Kaplan. Instead, he's chased by a crop-dusting plane armed with machine guns. Managing to escape and get to Chicago, Thornhill finds that Kaplan has checked out of the hotel where he's been staying. But who does he find there instead?
You got it: Eve Kendall. Thornhill corners her; he wants some answers.
She sneaks away after warning Thornhill to stay away—to no avail, though. He follows her to an auction house, where she sits with Vandamm and Leonard through an auction in progress. Here Thornhill understands that Eve's sleeping with the enemy. He realizes that he's better off in police custody than chased by Vandamm, who still thinks he's George Kaplan.
Thornhill makes a hilarious scene at the auction as a way to make sure he gets carted away by police instead of by Vandamm's murderous cronies. To his surprise, though, the police lead him to the Professor, who explains that there's no George Kaplan; he's a decoy. He also lets Thornhill in on another secret: Eve's not working for Vandamm alone, but for the Professor and Friends. Now that he knows Eve's a double agent, Thornhill agrees to help save her, which means getting on a plane headed to Rapid City, South Dakota and pretending to be George Kaplan.
As part of the Professor's plan, Thornhill fakes his own death after Eve shoots a blank at him near Mount Rushmore. (She's trying to prove her "loyalty" to Vandamm.) There's an exciting chase (understatement of the century) across the top of Mount Rushmore and over the presidents' faces that leads to the heroes' final rescue and—of course—their happily ever after.