As the Flyer passes around a curve adjacent to rocks, Sundance jumps onto the center of one of the train cars - a dangerous stunt through the air - and after a moment's pause makes his way forward toward the engine. He jumps into the cab and pulls a gun on the Fireman (Don Keefer) and the Engineer, and orders the train to be slowed. Although initially frightened, the Fireman looks down the tracks and is pleased to see the notorious Butch in person, standing by the side of the tracks: "I'll bet that's ol' Butch himself." He gets off the train to observe: "I just thought I'd watch." Sundance quips: "Bring the kids, why don't ya?"

Outside the express car, Butch implores the agent inside to open it up and avoid getting hurt: "You're just gonna get yourself blown up if you don't open that door." The voice of Woodcock (George Furth), the stubborn, 'patriotic', and loyal agent for E. H. Harriman, the President of the Railroad, resists from inside the car, refusing to risk his job and let the outlaws have access to the safe inside the car. Butch pleads: "Do you think E. H. Harriman would get himself killed for you - Woodcock?" To get at the safe and its money, an explosive dynamite charge blows a large hole in the wall of the railroad car, injuring Woodcock. Concerned, Butch revives the bloodied agent on the floor of the car: "Woodcock, you alright? Hey! Whatever Harriman's payin' ya, it ain't enough." More dynamite is used to open the safe, and although there isn't much money inside, Butch smiles: "Well, just so we come out ahead, that's the main thing."

The nearby town's Marshal (Kenneth Mars) vainly attempts to gather a posse together from an unresponsive audience, but the crowd knows better: "They're probably halfway to Hole-in-the-Wall already...Head 'em off? You crazy? We did that and they'd kill us." The camera slowly pans upward to the second floor balcony of Butch and Sundance's favorite brothel/saloon (Fanny Porter's), where the two sit on the porch - observing, listening, and drinking beers. Butch beams: "Boy, I just eat this up with a spoon." A send-off party is being celebrated for the saloon's piano player, who has enlisted to fight in the 1898 war against the Spanish ("Remember the Maine"). Butch has always dreamt of his own life as being heroic:

Butch: You know, when I was a kid, I always thought I was gonna grow up to be a hero.
Sundance: (pragmatically) Well, it's too late now.
Butch: What'd you say something like that for? You didn't have to say something like that.

After his futile attempts, the Marshal is left without a single volunteer to pursue the famed gang. Inspired by the jingoistic spirit, Butch is excited about the possibility of becoming a genuine military hero in the Spanish-American War:

Butch: Why don't we enlist - go fight the Spanish - you and me in the war? We've got a lot of things going for us: experience, maturity, leadership. I bet we'd end up officers. I'd be Major Parker...
Sundance: Parker?
Butch: Yeah, that's my real name - Robert Leroy Parker.
Sundance: No foolin'?...Mine's Longbaugh.
Butch: No foolin'? Long- what?
Sundance: Harry Longbaugh.
Butch: So you'd be Major Longbaugh. Whaddya say?
Sundance: You just keep thinkin', Butch. That's what you're good at.

Because of the changing times, an opportunistic bicycle salesman (Henry Jones) interrupts the Marshal and draws a more curious, interested crowd to peddle his newfangled 20th century merchandise:

Marshal: Well, whaddaya say?
Bicycle Salesman: (He moves up next to the Marshal) I say this, I say ladies and gentlemen, boys and girls, friends and enemies, meet the future.
Crowd member: The future what?
Salesman: The future mode of transportation for this weary Western world. Now I'm not gonna make a lot of extravagant claims for this little machine. Sure, it'll change your whole life for the better, but that's all.
Marshal: And just what in the hell do ya think you're doing?
Salesman: Well, you got the crowd together. That's half my job, so I just thought I'd do a little selling.
Marshal: Well, I'm trying to raise a posse here if you don't mind?
Salesman: I got a short presentation. (To the crowd) The Horse Is Dead. You'll see - this item sells itself.

After admiring the new 'mode of transportation' from afar, Butch is approached by one of the saloon women. Drunken Sundance announces that he will also go "hunting" for a woman - after running down a long list of qualifications: "Well, I think I'll get saddled up and go lookin' for a woman too...It shouldn't take more than a couple of days. I'm not picky, as long as she's smart and pretty, and sweet, and gentle, and tender and refined, lovely, carefree..."

In a nearby part of town that evening, primly-dressed mid-twenties schoolteacher Etta Place (Katharine Ross) with her hair in a bun returns to her farmhouse. [The scene mocks the cliched concept of the starched Western schoolmarm.] She lights a lantern, and undresses down to a white slip as she moves into her bedroom. The Sundance Kid, who has made a forced entry and waited for her in a corner of the room, surprises her there. His appearance - with a grin on his face - causes her to jump back in fright. Sundance commands her to keep slipping out of her clothes for him - at gunpoint:

Sundance: Keep going, teacher lady. (He reaches for his pistol and points it at her.) It's OK, don't mind me. Keep on going. (She nervously removes her outer slip.) Put down your hair. (She reaches back behind her head with both hands. Her hair falls to her shoulders.) Shake your head. (He examines her appreciatively. Cocking his gun, he threatens for her to undo the last remaining bits of clothing. She unbuttons her undergarment, revealing the flesh of her body. Then he unbuttons his holster, rises from his chair and approaches toward her with amorous intentions to force himself upon her.)
Etta: (Unafraid, she watches him approach. He begins to caress her.) (Sadly) Do you know what I wish?
Sundance: What?
Etta: (chiding) That once, you'd get here on time!

It is soon apparent that this is a game. She is his hot-blooded, renegade, twenty-six year old girlfriend - not a demure virgin. They are lovers that know each other very well. She wraps her arms around him as they embrace and kiss.

In a memorable wordless, frolicsome bicycle scene which has nothing to do with the film's plot, Butch appears outside their window the next morning riding one of the salesman's new-fangled bicycles of "the future." His voice, in the tone of melodramatic villains, intones from outside: "You are mine, Etta Place. Mine. Do you hear me? Mine. All mine. Your soft white flesh is mine. Soft. White." Both men vie for the attentions of Etta. His disembodied head (with a Charlie Chaplin-like bowler hat) glides past the window. Etta rises and stands at the front door, where Butch gestures for her to get on the crossbar.

Butch: Meet the future.
Etta: Do you know what you're doing?
Butch: Theoretically.

Butch tries out the latest newfangled invention, with Etta precariously perched on the handlebars, accompanied by Burt Bacharach's contemporary smash hit, the Award-winning song: "Raindrops Keep Fallin' On My Head" (sung by B. J. Thomas, and lyrics by Hal David). [The comical-lyrical "Bicycle Ride" sequence is like a music video dropped into the middle of the film.] During the fun ride in the golden rays of the dawn's sunlight, they ride downhill. As she watches from the loft of an old barn, he performs stunts to impress her, and ends up ungracefully tumbling onto the ground.

After their bike ride as they walk back to the house, Etta asks about his plans with Sundance:

Etta: You've come to get him for the Flyer?
Butch: Do you believe I'm broke already?
Etta: Why is there never any money, Butch?
Butch: I swear, Etta, I don't know. I've been working like a dog all my life and I can't get a penny ahead.
Etta: Sundance says it's because you're a soft touch and always taking expensive vacations and buying drinks for everyone and you're a rotten gambler.
Butch: Well, that might have something to do with it.

At the front of her house, Butch lightly kisses her on the cheek, and she ponders their relationship:

Etta: Butch? Do you ever wonder if I'd met you first we'd been the ones to get involved?
Butch: Well we are involved, Etta. Don't you know that? I mean, you are riding on my bicycle. In some Arabian countries, that's the same as being married.

Sundance emerges and finds Butch kissing and hugging her:

Sundance: Hey, what are you doing?
Butch: Stealing your woman.
Sundance: (scratching his butt) Take her, take her.
Butch: Well, you're a romantic bastard, I'll give you that.

The second robbery of the Union Pacific Flyer is less successful than the first. They again encounter stubborn - bruised and bandaged Woodcock guarding the safe in the express car: "Butch - you know that if it were my money, there is nobody that I would rather have steal it than you, but you see I am still in the employ of Mr. E. H. Harriman of the Union Pacific Railroad." A loud, oversized female passenger (Jody Gilbert) protests the delays and bulls her way over to the robbers:

I'm a grandmother and a female and I've got my rights!...You can bull all the others but you can't bull me. I've fought whiskey and I've fought gambling, and I can certainly fight you.

Sundance and Butch pretend to be hurting the woman and imitate her voice, tricking Woodcock to open up the car's door. The inept Hole-in-the-Wall train robbers use too much dynamite to blow up the safe, now reinforced and larger. They blow up the entire railroad boxcar - a startlingly real explosion with a deafening sound and flash of light. [It is a clever reversal of another Western cliche.] After the tremendous blast, they watch the pieces of paper money fly away and flutter around in the wind. Sundance laughingly jokes: "Do you think you used enough dynamite there, Butch?" Gang members scramble around and begin gathering up the loot that blows over the ground. In the distance, Butch notices an ominous black cloud from a second train locomotive and is bewildered: "Now what the hell is that?"

The President of the Railroad has anticipated their hold-up. Following the first train is another locomotive with a boxcar attached containing horses and a posse specifically formed to eliminate Butch and his gang. Before they can gather up the money, a formidable Superposse of a half-dozen men on horseback swiftly exits from the side of the boxcar - cued by the sounding of the train's whistle. Butch gives one look and senses trouble: "Whatever they're sellin', I don't want it!" Several members of the gang are shot down as they run for their horses. Sundance rates their shooting skill as Flat Nose and News are shot dead: "They're very good!" Even though the fleeing gang of four members splits in two directions to evade the determined posse, Butch and Sundance are the ones that are stalked. At the crest of a hill, they are upset that the entire posse is relentlessly following them: "What's the matter with those guys?" Their strained banter during the chase is wryly humorous:

Butch: I think we lost 'em. Do you think we lost 'em?
Sundance: No.
Butch: Neither do I.

To take refuge and hide out, they go to Fanny's brothel in town, where they commission a cherubic, pink-faced Sweet Face (Percy Helton) to provide an alibi to the posse about their whereabouts: "You seen us ride through town not five minutes ago. You do this right, I'll get you an old dog to kick." Sweet Face is positioned across the street on a porch, whittling on some wood. In a top floor room, Butch is caressed by blonde-haired prostitute Agnes (Cloris Leachman), while Sundance nervously looks out a curtained window at Sweet Face. The two fugitives watch with relief as Sweet Face converses with the posse and gestures down the street. Sundance leaves the room as Butch returns to Agnes. From opposite sides of the bed with their backs to each other, Butch removes his boots and both of them undress - Agnes speaks of her feelings for him:

You're the only real man I ever met, you know that Butch? It's not just because of all that money you got to spend on people. It's you. They way you're always looking to see am I happy or not. A lot of the other girls, they might want you just for when you got lots of money to spend on people. Me, I don't care about clothes and money and jewels and furs and things like that. Lots of the others girls do, but I never did...

With the sound of horses' hooves, they are alerted to the return of the posse. Sweet Face confesses and points up toward their window. When their decoy plan backfires, they must hurriedly slip out the second story window and climb down from the rooftops. They quickly subdue one of the posse's guards who is watching the group's horses. While Sundance gets their own horses for a quick getaway, Butch unties the posse's horses from the hitching post and starts shooing them away: "Go on, get out of here, you fat-headed beasts." But the enormous horses stand perfectly still, staring at him flailing his hands wildly in the air. Sundance returns with Butch's horse and warns his partner: "You're the fat-headed beast. Quit shouting!" Butch is dumbfounded that his efforts are ineffectual: "Boy, somebody sure trained 'em." [The sequence is another irreverent twist on Western cliches.]

Through thick dark woods, they attempt to avoid leaving a trail. From the crest of a hill, they watch for any sign of pursuit:

Butch: How long before you figure they're not after us?
Sundance: A while longer.
Butch: How come you're so talkative?
Sundance: Just naturally blabby, I guess.
Butch: I haven't done so much ridin' since I quit rustling. That's a miserable occupation. Dusk to dawn, no sleep, rotten food.

They suddenly stop, faintly noticing a slowly-moving, phantom-like glow in the darkness - they are the torches or lanterns of the posse following their path dead-on. Worried, they whisper to each other: "I couldn't do that. Could you do that? How can they do that?" They repeatedly ask each other the bewildering question - as they look over their shoulders and try to evade the Pinkerton posse:

Who are those guys?

They are endlessly pursued by the machine-like, impersonal, superhuman posse, often filmed with a long focal-length lens, while the two outlaws are frequently filmed in close-up. Certain that he has an infallible plan, Butch proposes that they double up on one horse, hoping that the riders will divide in different directions: "This'll work!" From another high vantage point, they look back at the route of the glowing posse. The glow separates into different paths, but only momentarily. The two glows move back together again into a single force: "They're not going for it. Who are those guys?"