Later, in a cool forest filled with ponderosa pines, the national park vehicle drives up. Unharmed, the "dead" Roger climbs out of the back of the station wagon. The camera tracks backward to reveal Eve standing next to her car. Both gaze at each other from a long shot across the subdued light amongst the trees. He approaches toward her, then she approaches toward him - they are reunited together and connected at last. Revealing her true identity (as a double agent) more than ever before, she is the first to apologize about lying to him, and she also expresses how his "harsh words...hurt deeply":

Eve: I wanted to tell you, I mean apologize.
Roger: No need. I understand. All in the line of duty.
Eve: I did treat you miserably.
Roger: I, uh, I hated you for it.
Eve: I didn't want you to go on thinking that I...
Roger: I, I used some pretty harsh words, I'm sorry.
Eve: They hurt deeply.
Roger: Naturally if I'd known.
Eve: I couldn't tell you.
Roger: Of course not.
Eve: Could I?
Roger: No, I guess not.

She explains how she originally became involved with Vandamm, almost whimsically ("I guess I had nothing to do that weekend..."), to help the intelligence agency. As she discusses her devotion to the cause with the Professor, she provides the context for his awakening and the budding, committed relationship (of marriage) that she will soon develop with him:

Eve: I guess I had nothing to do that weekend, so I decided to fall in love.
Roger: That's nice.
Eve: Eventually, the Professor and his Washington colleagues approached me with a few sordid details about Philip. They told me that my relationship with him made me uniquely valuable to them.
Roger: Mm-mmm. So you became a Girl Scout, huh?
Eve: Maybe it was the first time anyone ever asked me to do anything worthwhile.
Roger: Has life been like that?
Eve (assenting): Mm-mmm.
Roger: How come?
Eve: Men like you!
Roger: What's wrong with men like me?
Eve: They don't believe in marriage.
Roger: I've been married twice.
Eve: See what I mean?
Roger: Now I may go back to hating you: it was more fun. (They kiss passionately.)

Roger wants to prolong their goodbye after being healed (from being "critically wounded") by their revelations, but she feels she must get back to Vandamm and continue on with her work before the spies got suspicious:

Eve: Goodbye, darling.
Roger: Wait a minute, not so soon.
Eve: I've got to get back to the house and convince them that I took the long way around so nobody would follow me.
Roger: Couldn't we stand like this for just a few hours?
Eve: Just this time, you're supposed to be critically wounded.
Roger: I never felt more alive.
Eve: Whose side are you on?
Roger: Yours always, darling.

The Professor honks the vehicle's horn to alert them to separate. Roger tells her that after Vandamm has left the country, he wants to spend more time with her in private: "You and I are going to get together and do a lot of apologizing to each other in private."

Roger then learns that he has been duped by the Professor, and her duty demands that she leave America that evening with Vandamm and fly away with him on his plane. Her excuse for staying with Vandamm and learning more information about his operation is that she is a wanted "fugitive."

Eve: Well, he has told you, hasn't he?
Roger: Told me what?
The Professor: Miss Kendall, you've gotta get moving.
Roger: Wait a minute. What didn't you tell me?
Eve: Why didn't you?
The Professor: She's going off with Vandamm tonight on the plane.
Roger: She's going off with Vandamm?
The Professor: That's why we went to such lengths to make her a fugitive from justice. So that Vandamm couldn't very well decline to take her along....I needn't tell you how valuable she can be to us over there.

Thornhill's reaction is one of furious outrage that she would continue to prostitute herself ("dirty business") and that he was lied to:

Roger: I don't like the games you play, Professor.
The Professor: War is hell, Mr. Thornhill, even when it's a cold one.
Roger: If you fellas can't lick the Vandamms of this world, without asking girls like her to bed down with him and fly away with him and probably never come back, perhaps you ought to start learning how to lose a few cold wars!
The Professor: I'm afraid we're already doing that.

Eve rushes off to her car and drives away, causing Thornhill to protest, but he is knocked unconscious by a park ranger's punch to the jaw.

Through radio news reports, government agent 'Kaplan' is reported to be in critical condition in the Rapid City Hospital following the spectacular "untimely shooting" scene at Mount Rushmore. But Thornhill is furious for being confined in a locked hospital room. Although he is provided with fresh clothes by the Professor for a couple of days, and promised a "pint" of bourbon (the drink forced on him earlier at Glen Cove, although he is an avowed gin-drinker), he executes an escape. Using the hospital window ledge to climb over to the next hospital room, he surprises its female occupant. (In a brief humorous vignette, she cries "STOP," but after putting on her glasses purrs another more inviting "Stop.")

He leaves the hospital and takes a taxi cab up to the driveway of Vandamm's modernistic house in view of Mount Rushmore, to prevent Eve from leaving. Vandamm's home is an imposing, cliffside glass retreat with cantilevered main rooms (set near a small concealed airplane runway). After climbing up the beams of the residence, he first overhears Vandamm reassuring Eve (about her shooting of Kaplan), and then listens to their plans about leaving shortly (on their plane landing in 10 minutes).

Then, he overhears that Leonard, speaking privately with innuendo to Vandamm, distrusts Eve:

Leonard: I know how terribly fond you are of Miss Kendall....You must've had some doubts about her yourself, and still do...Why else would you've decided not to tell her that our little treasure here has a bellyful of microfilm?...Call it my 'woman's intuition', if you will, but I've never trusted neatness. Neatness is always the result of deliberate planning.
Vandamm: She shot him in a moment of fear and anger. You were there yourself. You saw it.

Leonard has read through her "neat and tidy" deception. Vandamm thinks Leonard has misinterpreted and feels (homosexual?) jealousy toward Eve for Vandamm's attention: "You know what I think? I think you're jealous. No, I mean it. I'm very touched, very." But Leonard melodramatically and vindictively demonstrates that Eve used a gun with blanks. He fires Eve's gun at Vandamm - leaving him unharmed: "It's an old Gestapo trick. Shoot one of your own people to show that you're not one of them. They've just freshened it up a bit with blank cartridges." Vandamm is horrified to learn that Eve is a double agent, and punches Leonard into a chair. But Vandamm also plots to kill her by throwing her from their departing airplane:

This matter is best disposed of from a great height - over water.

Eve rejoins them in the living room for a drink of warm champagne, suggesting (with tongue-in-cheek humor referring to the final cliff-hanging sequence) that "over the rocks will be all right."

After overhearing their dastardly plan to kill Eve, Thornhill realizes he must save her with a warning. He scribbles a note on one of his monogrammed (R O T) matchbooks: "They're onto you - I'm in your room" and drops it down to the living room near where she is seated. After alerted, Eve makes an excuse that she has left her earrings in her room. She returns and finds Roger there - he tells her that her life is in grave danger and that she must not board the plane. Then, they depart for the waiting plane on the concealed landing strip behind the house (to transport Vandamm and his antique art object that he bought at the auction - with microfilm secretly enclosed). Roger is discovered by the housekeeper Anna (Nora Marlowe) when she notices his reflection in the television set. Holding him at gunpoint, Roger is detained. Nervously, Eve is escorted by Vandamm and Leonard to the plane - she turns back frequently and expects Roger's rescue.

As they are about to board the aircraft, two gunshots are heard from the house (fired by housekeeper from the gun with blanks). During the distraction, Eve bolts from Vandamm after snatching the art statue/microfilm and jumps into Roger's stolen getaway car that he has driven dangerously close to them. The two abandon the car when they cannot open the locked driveway gate to the estate - there is a memorable image of the two of them captured in the car's headlights. They escape by foot from Leonard and Valerian, running through the woods and finding themselves at the top of Mount Rushmore. They peer over the massive tops of the heads of the Presidents, directly atop Thomas Jefferson.

[The scenes on the gigantic heads at Mount Rushmore National Monument were actually shot in a studio on a simulated replica of the site (with mockups of the faces and massive rear projections), since the Department of the Interior wouldn't allow filming at the actual site.]

In the cliffhanger climax, they are chased and forced to climb down the monument, across the faces of the historical, ex-President figures on Mount Rushmore. Chased by dangerous killers during the descent, they are compelled to start down the sculptures (between Washington and Jefferson) and hang off the colossal monument. As they dangle there precariously, Thornhill asks Eve to marry him after mentioning his two previous marriages:

Roger: If we ever get out of this alive, let's go back to New York on the train together, all right?
Eve: Is that a proposition?
Roger: It's a proposal, sweetie.
Eve: What happened to the first two marriages?
Roger: My wives divorced me.
Eve: Why?
Roger: Well, I think they said I led too dull a life.

One of the killers Valerian (who has climbed down on the other side of Washington's face) jumps Roger with a knife, but Roger throws him over the edge during the scuffle. While they are struggling, Leonard (who has emerged on the far side of Lincoln's face and crossed over) snatches the statue from Eve's hands and throws her down the rock wall, where she clings for life from a thin ledge. Roger reaches down to pull up her outstretched hand, tenuously hanging on to the woman he loves above the abyss (and literally wants to become attached to). With his other hand, he clings to the rock ledge for support. He appeals to Leonard above him for help, but the evil spy brutally grinds and crushes Roger's hand with his foot. A shot rings out, the statue drops and shatters (revealing the microfilm inside), and Leonard falls to his death from a Sergeant's bullet. The Professor has arrived just in time to capture and arrest Vandamm. The witty mastermind spy chides the police after his partner has been shot: "That wasn't very sporting, using real bullets."

In a clever transition expressing their physical survival and their new real relationship, Roger continues to struggle in hauling Eve to safety from the rock ledge, and then - CUT - is abruptly seen pulling her into his upper train berth in a Pullman car: "Come along, Mrs. Thornhill." They are last seen on their honeymoon as they bed down for the night in their private train compartment.

Eve: Oh Roger, this is silly.
Roger: I know, but I'm sentimental.

They are returning to their starting point, going east on the train. The film's final shot is blatantly phallic - their train glides into a tunnel.