Bridge construction proceeds, as the British enlisted men are engaged in building a "proper bridge" for their single-minded commander. Nicholson is pleased with their job: "Fine job our chaps are doing. Really first-rate." Although Nicholson believes the construction is a "good idea" because it builds the men's morale and re-establishes discipline, Clipton doubts the bridge's benefits, because it ultimately serves the Japanese purpose and could be considered treasonous:

The fact is, what we're doing could be construed as, forgive me sir, collaboration with the enemy. Perhaps even as treasonable activity...Must we work so well. Must we build them a better bridge than they could have built for themselves?

Nicholson dismisses the notion that the British are aiding the enemy and contributing to the Japanese war effort, interesting only in showing the Japanese savages what morally and intellectually-superior British ingenuity and knowledge can accomplish. His British-built bridge endorses the idea that the Japanese are incapable of constructing such a monument. He also indulges himself with a fanciful vision of the bridge after the war is over - a bridge that demonstrates the superiority of the British soldier to future generations:

One day the war will be over. And I hope that the people that use this bridge in years to come will remember how it was built and who built it. Not a gang of slaves, but soldiers, British soldiers, Clipton, even in captivity.

The international commando team chooses a fourth member, a young Canadian named Lt. Joyce (Geoffrey Horne), and practice parachute jumps are considered for Shears. He is also shown the new L pill - to be used to prevent being taken captive by the enemy, or to be used if hurt or wounded on the trek, making the objective of the mission difficult to achieve: "L for lethal. Instantaneous, painless. Much better than the older ones. For capture, of course."

Force 316 parachutes into the jungle (one of the four, Chapman, is killed landing in the trees), and the sabotage unit must change their route to the Kwai Bridge by swinging far to the north through heavy jungle to avoid frequent Japanese patrols. As they struggle through the dense jungle, rice fields, and leech-infested muddy rivers on their trek, they use pretty, native women-bearers. As one of the Siamese-speaking bearers removes leeches from Shears' back, he quips with an oft-used cliche:

What's a nice girl like you doing in a place like this?

After a fortuitous repair of the radio, they receive a vital transmission, suggesting the destruction of the bridge just as a VIP train is passing over it - if they can reach the bridge by the evening of the 12th of May:

One: Original bridgeworks reported abandoned. New construction downstream from first site.
Two: Enemy intends to open railway with passage of special train, Bangkok to Rangoon with troops and VIP estimated to arrive target, AM, 13th.
Three: You should synchronize demolition with pass of this train.
Four: Good hunting. Have fun.

The same time pressures confront Nicholson and the bridge builders with a "crisis." The Colonel's ultimate folly is to relentlessly drive and order his regiments to work harder than Saito would have. In a remarkable about-face turnabout, he sacrifices and forfeits the Geneva Convention rights that he had earlier defended, and even pressures Clipton to release some of the sick in the hospital (including some he calls "malingerers") for some "light work" to help in the effort:

Nicholson: We're not going to finish the bridge on time...We haven't the manpower, that's all. I've asked the officers to lend a hand and they've agreed. But even that won't do it.
Clipton: You mean the officers are going to work on the bridge?
Nicholson: Yes. I explained the situation to them and they volunteered to a man, but it's not enough.
Clipton: Why didn't you ask Saito and some of his men?
Nicholson: Wouldn't dream of it! No, this is our show. We must make the most of our own resources. As a matter of fact, that's what I came to talk to you about. The sick list.

A bat flies above a beautiful waterfall, as the women and men of the commando sabotage mission pause to bathe and clean up. The team must fire upon a surprise contingent of Japanese on patrol, sending thousands of bats into the sky and darkening it. Warden and Joyce pursue one of the enemy on foot through the jungle. When Joyce hesitates to kill his young counterpart with his knife, Warden stabs the man to death in the chest. Next to the body on the ground, there is a picture of the soldier's loved ones. Warden suffers a "superficial" wound - a chipped bone in his left foot. The bloody wound becomes more serious than originally thought, and Warden falls further and further behind during the trek.

Although Warden orders the group to leave him behind, Shears refuses and calls for a stretcher, emphatically expressing the insanity of following rules in war ("I don't care about your bridge and I don't care about your rules"). He expresses an ideological belief in humanism ("the only important thing is how to live like a human being"):

Warden: You'll go on without me. That's an order. You're in command now, Shears.
Shears: You make me sick with your heroics. There's a stench of death about ya. You carry it in your pack like the plague. Explosives and L pills. They go well together, don't they? And with you, it's just one thing or the other: 'Destroy a bridge or destroy yourself.' This is just a game, this war. You and that Colonel Nicholson, you're two of a kind. Crazy with courage. For what? How to die like a gentleman. How to die by the rules when the only important thing is how to live like a human being. I'm not going to leave you here to die, Warden, because I don't care about your bridge and I don't care about your rules. If we go on, we go on together.

In the late afternoon of May 12th from a high mountaintop above the Kwai River, Shears is the first to view the completed bridge site. Warden observes: "I can't understand it. It's such a solid, well-designed job. Not like the temporary bridges the enemy usually throws together." Through a scope, Joyce notices a "British officer working down there on his knees" with the "poor devils." Ironically, the officer is not being verbally or physically abused, but is putting the final touches on his own creation.

From a closer view, a commemorative sign/plaque is erected on the completed bridge, hammered in place by Colonel Nicholson as a symbol of pride. It identifies the builders of the impressive, marvelous, technologically-superior structure:

Meanwhile, the Americans plan to manually set explosives to blow up the bridge as a train carrying Japanese dignitaries crosses the next day. That night, the charges will be set against the piles about three feet under the water, with the main wire hidden and run downstream to the plunger. A raft is built to float the explosives to the bridge and the commando unit prepares itself by putting on camouflage mud.

Nicholson and Saito meet mid-span on the beautifully engineered, completed bridge as the sun sets, exchanging views and reflecting on its magnificent beauty. As Saito stands behind him, Nicholson leans over one of the guard rails and looks out over the river while delivering a personal reverie about his years of military service. He reflects on his "good life," particularly as a regular officer in India (the ultimate destination of the Japanese railroad route that he has helped to construct):

Saito: Beautiful.
Nicholson: Yes, beautiful. A first-rate job. I had no idea it would turn out so well.
Saito: Yes, a beautiful creation.
Nicholson: I've been thinking. Tomorrow it will be 28 years to the day that I've been in the service, 28 years in peace and war. I don't suppose I've been at home more than ten months in all that time. Still, it's been a good life. I love India. I wouldn't have had it any other way. But there are times when suddenly you realize you're nearer the end than the beginning. And you wonder, you ask yourself, what the sum total of your life represents, what difference your being there at any time made to anything, or if it made any difference at all really. Particularly in comparison with other men's careers. I don't know whether that kind of thinking is very healthy, but I must admit I've had some thoughts on those lines from time to time. But tonight -- tonight! (He accidentally drops his stick into the river.) Blast! I must be off. The men are preparing some sort of entertainment.

While the men celebrate their tremendous accomplishment, a humorous stage show featuring hula-skirted dancers in front of a miniature replica/model of the bridge, the saboteurs float a raft with explosives down to the bridge under cover of night. In his quarters, kimono-clad Saito prepares himself for ritualistic hari-kiri the next day, and writes out a suicide note. Explosives are set on the pilings of the bridge by the commandos. (The only sounds on the soundtrack are the footsteps of Japanese guards above them on the planking, the lapping of the water against the bridge, and the distant sounds of the entertainment.) Nicholson congratulates his men on the completion of their work, understanding that they might be "let down":

It is quite understandable. It's a very natural reaction. But one day, in a week, a month, a year, on that day when God willing, we all return to our homes again, you're going to feel very proud of what you have achieved here in the face of great adversity. What you have done, should be, and I think will be, an example to all our countrymen, soldier and civilian alike. You have survived with honor, that and more, here in the wilderness. You have turned defeat into victory. I congratulate you. Well done.

The explosives are wired to a plunger placed downstream where Lt. Joyce is positioned and hidden behind a large rock. As dawn comes, the commandos are horrified to realize "the river's gone down" - the low water level exposes the charges on the pilings and some of the wire leading to the plunger.

As morning dawns, Saito cuts a ribbon and the soldiers march across the bridge, whistling "The Colonel Bogey March" as they are transferred to a new camp. Nicholson, Clipton, Saito, and the sick men wait for the arrival of the train, due in five or ten minutes. The exciting finale pits Nicholson in a misguided loyalty to his bridge against his own Allied commandos. As he proudly walks over the bridge and leans over the railing, he notices wires coming from the bridge but doesn't quite understand their significance. Saito, who has taken out his suicidal knife, is alerted by Nicholson: "There's something rather odd going on. I think we'd better have another look around before that train comes across."

The two walk down to the water's edge, as the sounds of the chugging and whistling train are heard in the distance coming closer and closer. Warden exclaims: "Our own man!" as Nicholson follows the dynamite leads along the shoreline toward the plunger. The two wade across the river, where Nicholson realizes his fears are confirmed: "I was right. There is something going on." He pulls up the detonation wire, and hand-over-fist follows its path to its origin.

From the other bank, Shears holds up his knife, indicating what the Canadian commando must do:

You've got to do it, boy. You've got to do it now!

Nicholson - whose pride in his work takes priority over other political/military considerations, attempts to alarm Saito and everyone else that the bridge is mined. He asks for a knife to cut the wires: "Colonel Saito. Have you a knife? I just realized. The bridge has been mined." From his hiding place, Lt. Joyce rushes forward and stabs Saito to death in the back. As Nicholson wrestles Joyce to the ground in the shallow water, he is told:

Joyce: Officer, sir. A British officer. We're here to blow up the bridge, sir!
Nicholson: Blow up the bridge?
Joyce: Yes, sir. Yes, sir. British commando orders, sir.
Nicholson (in disbelief): Blow up the bridge!?

Nicholson struggles with Joyce to prevent him from pushing the plunger, as both Warden and Shears from their different vantage points shout out to Joyce: "Kill him!" Shears dives into the water from the other shore to reinforce the position. He swims across the water in an attempt to reach and kill Nicholson, and then detonate the bridge. Joyce is killed first by one of the shots (from Warden's own weapon!) - followed by a brief camera reaction shot of Joyce's lover - one of the Siamese porter-ladies on the hillside. Next, Shears is wounded in the leg by a shot from a Japanese guard, followed by another quick reaction shot of the face of his Siamese lover, and then a second fatal shot (again from Warden's gun). Mortars fired from Warden on the hillside kill more of the Japanese guards.

Nicholson finally realizes he is blocking his own Allied side's operation to blow up the bridge. He gasps:

What have I done?

At the exact moment when the train is passing over the bridge, he is hit by shrapnel from the mortars. Nicholson staggers around semi-consciously, and falls mortally wounded on the dynamite plunger. He blows up his beloved bridge and sends the trainload of Japanese soldiers into the river. The pleasing-to-look-at bridge crumbles and collapses.

[The motives behind the detonation of the bridge are unclear and ambiguous in the film's dramatic ending. His fall onto the detonator - was it accidental or intentional?]

Always the cold and calculating soldier, Warden turns toward the only survivors of the commando raid, the women-bearers. He feels guilty and begs their forgiveness for using mortars - and for using his own rifle to kill two of the commandos (Joyce and Shears), his men:

I had to do it. I had to do it. They might have been captured alive. It was the only thing to do.

He throws the mortar launcher away in disgust. The total irony of war is best emphasized in the conclusion of the film, as a horrified Clipton, the spokesperson for an 'outsider's' point of view, despairingly observes the tremendous scene of human and physical wreckage, the collapsed bridge and corpse-littered scene:

Madness!...Madness! Madness!

The commemorative bridge plaque floats away amid the debris of dead men and a ruined bridge. The camera pans upward and back to reveal the magnitude of the awful, foolish, and mad destruction, but as it pulls further away, the carnage is lost in the jungle. The film ends as it began - a sole hawk flies overhead - without any deliberate, clarifying postscript or expository conclusion.