Upon the train's arrival in Chicago, Roger is disguised as a uniformed Red Cap porter and he carries Eve's luggage so that he can pass waiting police. On the platform, she is detained and questioned by the police but is not considered a suspect. She offers to phone Kaplan and arrange an urgent meeting with him as soon as they get inside the station. She suggests that Roger change his clothes in the men's room while she phones to make arrangements to rendezvous with Kaplan. Roger remarks:

You're the smartest girl I ever spent the night with on the train.

When they are almost inside the station, a Red Cap in his underwear alerts police, sending detectives racing through La Salle Street Station grabbing Red Caps everywhere. Meanwhile, Roger is changing and shaving in the restroom with Eve's miniature razor. Eve makes a call from a phone booth - the camera pans down the length of booths - she actually calls Leonard at the far end of the block of booths and receives instructions. When Roger returns, the treacherous Eve lies to him about a conversation with Kaplan [in fact, she has arranged for an attempt on his life]. The mysterious Kaplan has agreed to a meeting with Thornhill at the Prairie bus stop at a Highway 41 crossroads about 1 1/2 hours Greyhound Bus ride from Chicago. When they part at the Chicago station, he places his hand on her hand with the luggage as they say goodbye.

In the film's most-renowned and brilliant sequence, the crop-dusting sequence, Roger is lured into the flat countryside (of neighboring Indiana) by enemy spies on the pretext of meeting and connecting with the fabled Kaplan - his non-existent double. The dapper businessman arrives by bus at a barren road-crossing out in wide-open farm country surrounded by plowed-up dirt and cornfields, incongruously dressed in a neat suit in bright sunlight. [The actual filming site was located north of Bakersfield, CA, outside the towns of Wasco and Delano, just east of the intersection of Corcoran Road and Garces Highway (155).] He is entirely exposed and vulnerable - a modern, urban individual without any amenities or artificial resources - there isn't even background music on the soundtrack until the climax of the set-piece. Surrealistically, suspense slowly builds as cars pass through the desolate area. A truck sprays him with road dust. A car drops a man on the other side of the road from him to wait for a bus - is this man Kaplan? A buzzing crop-dusting plane is engaged in dusting a nearby field. The man remarks that it is odd to have a small plane crop-dusting a crop on a field devoid of crops:

That's funny...That plane's dustin' crops where there ain't no crops.

After the man boards a bus (a symbol of civilization) and it drives away (leaving Thornhill defenseless), the distant, innocent and harmless crop-spraying plane immediately and without warning terrorizes him, swooping down like a bird of prey out of a clear blue sky. It flies almost at ground level as it sprays machine-gun fire. Thornhill ducks for cover from the strafing attack, but there is nowhere to hide and no way to defend himself in the vast expanse of the setting - it is the third vicious attack on his life. The plane circles and returns a few times as he fails to flag down and stop a car.

Thornhill runs for cover in an open cornfield, but the bi-plane showers him with a load of poisonous, white powdery pesticide to flush him out. He returns to the road and runs in front of an approaching semi-trailer Magnum Oil truck - flagging it down and forcing it to stop. He falls under the gasoline truck's front bumper as the plane uncontrollably crashes into the truck's gas tank. After a terrific explosion, the two truck drivers shout that they should run away to avoid harm from more explosions. Other drivers in pickup trucks stop to watch the fireball, giving Roger an opportunity to jump into one of the onlookers' vacated trucks and head back to Chicago.

The police find the abandoned truck on a Chicago street. Roger enters the Ambassador Hotel and at the desk asks for Mr. Kaplan's room. He is shocked to learn that Kaplan checked out early that morning on his way to Rapid City, South Dakota - almost two hours before Eve allegedly claimed to have talked to him. Suddenly, he is doubly surprised when he notices Eve in a dark red gown crossing the lobby - she is also checked into the hotel. Wise to her duplicity, he follows her to her fourth floor room and walks coldly by her when she lets him in:

Roger: Surprised?
Eve: Yes.
Roger: No getting rid of me, is there?

She runs and hugs him, happily relieved to see him knowing that he could have been killed by her treachery. But he doesn't reciprocate - this time, he holds his hands around her head (the same way he held her in the seduction scene on the train), but he doesn't quite touch her. As she mixes drinks for them, he confronts her with the set-up she prepared for him in the attack in the open field. Ambiguously and deceptively, Eve cooly avoids his questions without confessing to any complicity. Roger sarcastically toasts their "togetherness" and their relationship:

Roger: To a long and lasting friendship, meaning from now on, I'm not going to let you out of my sight, sweetheart.
Eve: I'm afraid you'll have to.
Roger: Oh no.
Eve: I do have plans of my own, you know, and you do have problems.
Roger: Well, wouldn't it be nice if my problems and your plans were somehow connected? Then we could always stay close to each other and not have to go off in separate directions. Togetherness, you know what I mean?

Eve receives a phone call and takes down an address for a later business meeting. She calmly asks a big "favor" of him - she pleads for him to leave immediately and never to see her again:

I want you to leave right now. Stay far away from me and don't come near me again. We're not going to get involved. Last night was last night and it's all there was and that's all there is. There isn't going to be anything more between us. So please, goodbye, good luck, no conversation, just leave.

Then she relents and accepts his invitation for a final dinner together - if the hotel valet will clean his dusty suit. He calls for valet service (and removes his clothing for re-pressing):

Roger: Now, what can a man do with his clothes off for twenty minutes? Couldn't he have taken an hour?
Eve: You could always take a cold shower. (She helps him remove his suit jacket.)
Thornhill: That's right. When I was a little boy, I wouldn't even let my mother undress me.
Eve: You're a big boy now.
Thornhill: Yes, tell me. How does a girl like you get to be a girl like you?
Eve: Lucky, I guess.
Thornhill: No, not lucky. Naughty, wicked, up to no good. Ever kill anyone? Because I bet you could tease a man to death without half trying. So stop trying, huh.

He pretends to take a shower (whistling 'Singin' in the Rain') as she sneaks out of the room, then manages to get her destination's address by rubbing a pencil over the impression left on the next sheet of notepad paper. He follows Eve to a chic, 1212 North Michigan Avenue address - the site of a crowded art auction. There, he finds her with her supposed lover, the fake 'Townsend' (Vandamm) and his henchman Leonard. Vandamm stands in the front row, with Eve seated at his side. He caresses the back of her neck with his hand (a reminder of Thornhill's own touching of her head). After walking up the middle aisle during the bidding session, Roger confronts the trio:

The three of you together. Now, that's a picture only Charles Addams could draw.

[Charles Addams was a famous cartoonist for the New Yorker magazine, famous for his macabre drawings and his witty - but bizarre - sense of humor. His most popular characters were spun-off and became the basis for both a TV sitcom series, The Addams Family, and related feature films.]

Thornhill sarcastically tells them that his art specialty is the "art of survival." When the phony 'Townsend' (Vandamm) learns that Thornhill was in Eve's hotel room and followed her to the auction, he removes his loving hand from her neck and shoulder. With biting sarcasm toward Eve as the bidding for a Pre-Columbian art object commences, Roger refers to both the "sculpture" and to Eve (and her betrayal):

I didn't realize you were an art collector. I thought you just collected corpses...I'll bet you paid plenty for this little piece of sculpture...She's worth every dollar of it, take it from me. She puts her heart into her work. In fact, her whole body.

The auctioneer identifies Mr. Vandamm as the successful bidder for the art object [to be used later to hide microfilmed secrets - director Hitchcock's typical MacGuffin]. Vandamm then criticizes Roger for his unconvincing overacting - Vandamm is still convinced that Thornhill is Mr. Kaplan:

Vandamm: Has anyone ever told you that you overplay your various roles rather severely, Mr. Kaplan. First, you're the outraged Madison Avenue man who claims he's been mistaken for someone else. Then you play the fugitive from justice, supposedly trying to clear his name of a crime he knows he didn't commit. And now, you play the peevish lover, stung by jealousy and betrayal. It seems to me you fellows could stand a little less training from the FBI and a little more from the Actor's Studio.
Roger: Apparently, the only performance that will satisfy you is when I play dead. [Ironically, Thornhill's next acting role will be to "play dead."]
Vandamm: Your very next role. You'll be quite convincing, I assure you.
Roger: I wonder what subtle form of manslaughter is next on the program. Am I to be dropped into a vat of molten steel and become part of a new skyscraper, or are you going to ask this female to kiss me again and poison me to death?

Eve rises in anger to strike him, but sits down after Roger describes her unfeeling act: "Who are you kidding? You have no feelings to hurt." (Roger doesn't notice that she has tears in her eyes.) The Professor is in the audience observing their discussion. Roger threatens Vandamm with giving himself up to the police: "Something seems to tell me I've got a much better chance of survival if I go to the police."

Roger stalks away, but all of the exits are covered by Vandamm's henchmen. Sitting down in the audience, he watches Eve leave with Vandamm. Thornhill deliberately calls attention to himself with erratic bidding, questioning the authenticity of the art works, and heckling the auctioneer - a means of escape by disrupting the auction. [Again, Roger uses a crowd as a temporary means of cover and escape.] Finally, he bids an enormous sum for an antique, provokes a fight when asked to leave, and is arrested by summoned police. He tells one of Vandamm's henchmen as he is dragged away: "Sorry, old man. Too bad. Keep trying."

In the police car, (taking him first North on Michigan Avenue and then West to the airport), Roger identifies himself as Roger Thornhill - the UN killer. Feeling that he will be safest in police custody, he demands to be taken to police headquarters. He claims he is "a dangerous assassin - a mad killer on the loose." Instead of taking him to jail, the police are instructed by carphone to take him to Chicago's Midway Airport, where he is met by the Professor in the Northwest terminal. The Professor identifies himself as a government agent - who is forced to help him because the CIA's plans have become endangered:

FBI, CIA, ONI, we're all in the same alphabet soup.

The Professor explains that Thornhill has a plane to catch to Rapid City, South Dakota, to follow Vandamm and his mistress (Roger labels her a "treacherous little tramp"). Thornhill learns that Vandamm is trying to take microfilmed government secrets out of the country:

The Professor: Oh, you could say he is a sort of importer/exporter.
Roger: Of what?
The Professor: Oh, government secrets perhaps.

Vandamm has a mountaintop retreat there near the Mount Rushmore monument - often referred to as a "Frank Lloyd Wright" structure (actually a modernistic, MGM studio-created design since it would have been impossible for a structure of this size to hang on the cliff-edge within the Park) - his jumping off point to leave the country the next evening. Thornhill is told that George Kaplan never existed - he is only a decoy created by the American intelligence agency to divert attention away from a real CIA agent. Roger is asked to play out the character of Kaplan for another 24 hours - to 'become' the person he has inadvertently given flesh-and-blood status to - in order to help save the life of the endangered agent. (The plane's engines drown out some of their conversation.) Roger insists that he will no longer be a decoy. He lists his priorities for the Professor:

I'm an advertising man, not a red herring. I've got a job, a secretary, a mother, two ex-wives and several bartenders dependent upon me. And I don't intend to disappoint them all by getting myself slightly killed.

But then the Professor reveals the identity of the endangered US government's double agent working undercover - Eve Kendall:

The Professor (referring to Eve): If I thought there was any chance of changing your mind, I'd talk about Miss Kendall, of whom you so obviously disapprove.
Thornhill: Yes, for using sex like some people use a fly swatter.

Eve is also Vandamm's moll/mistress, now threatened with being exposed and having her cover blown by his actions. Horrified by the thought, Thornhill reluctantly agrees to go to Rapid City, follow Vandamm, and pretend to be Kaplan in order to save Eve's life. He thereby shows his acceptance of responsibility and commitment in his relationship with her. The light of the plane suddenly illuminates his face - to symbolically indicate his enlightenment, his connection to her, and his emotional empathy for her.

The impressive carved-stone mountainside of Mount Rushmore (a proud symbol of democracy and order - but in this context a setting for betrayal) fills the screen the next morning. There, Vandamm (with Eve) has agreed to meet Thornhill, finally accepting himself as George Kaplan, at the Mount Rushmore Monument cafeteria (at the base of the monument) where Roger has purchased a cup of coffee. With the goal of preventing Eve from leaving the country with Vandamm and to "restore her to Vandamm's good graces," Roger offers to let Vandamm leave the country without interference (without reporting Vandamm's location to his superiors) in exchange for Eve:

I want the girl to get what's coming to her. You turn her over to me and I'll see there's enough pinned on her to keep her uncomfortable for the rest of her life. You do that and I'll look the other way tonight.

Vandamm refuses - and acting outraged, Roger grabs Eve and pulls her toward him. In order to prevent a suspicious and jealous Vandamm from suspecting that she is on the side of the government, Eve pulls out a gun (loaded with blanks) from her handbag and fires two shots at him in the cafeteria. [One of the film's most obvious faux pas is in this scene - a young boy in the background covers his ears before the loud gunshot.] She then rushes alone from the building and drives away. When Thornhill collapses, apparently critically wounded, both a stunned Vandamm and Leonard choose to avoid getting involved. Thornhill is quickly aided by the Professor and carried by stretcher to a green park vehicle.