The Japanese Army forces World War II POWs to build a strategic bridge in Burma.

In World War II Burma, after they are captured by Japanese troops during World War II, British commander Col. Nicholson and his troops march into Prisoner of War Camp 16 whistling their regimental tune. Their crisp arrival is wryly observed by Shears, an American sailor who bribes a guard to transfer him from the burial detail to the infirmary. When the camp's commander, Col. Saito, imperiously informs the new prisoners that they will all be expected to work on building a railroad that will connect Bangkok to Rangoon, Nicholson protests that under the regulations of the Geneva Convention, all officers are exempt from manual labor. Afterward, Nicholson goes to the infirmary to visit Jennings, one of his wounded men. There Maj. Clipton, the camp's medical officer, introduces the colonel to "Commander Major" Shears. When Jennings proposes escaping, Nicholson counters that he was ordered by headquarters to surrender, and therefore escaping would constitute a military infraction. Incredulous at the colonel's naïveté, Shears retorts that escape is their only chance to avoid the death sentence of forced labor. The following day, Saito announces that all the men, including officers, will work on building a bridge across the River Kwai. When Nicholson defiantly waves a copy of the Geneva Convention, Saito slaps him across the face with it and flings it to the ground. Nicholson still refuses to let his officers perform manual labor, and after the other men march off to work, Saito calls for a machine gun and threatens to gun down all the officers. Watching in horror, Clipton runs out of the infirmary and protests that he and his patients have seen everything and will be witnesess to murder if Saito orders the gunners to fire. Saito then changes his mind and forces the officers to stand for the entire day in the merciless sun. Afterward, Saito locks Nicholson in "the oven," a crude metal shed, and imprisons the other officers in the "punishment hut." As the troops encourage Nicholson with a rendition of "For He's a Jolly Good Fellow," Shears, Jennings and a prisoner named Weaver escape into the jungle. Jennings and Weaver are gunned down by the guards, who then pursue Shears to a ridge above the river and shoot him. The guards assume that Shears is dead after he plunges into the river, but he survives and makes his way down river to the shore. Later, Saito summons Clipton and asks him to tell Nicholson that unless he cooperates, the patients in the infirmary will be forced to work. Confronted with the ultimatum, Nicholson still refuses to comply on the grounds that "it is a matter of principle." With only two months left before the May first deadline for the completion of the bridge, Saito, frustrated by the slow progress, takes command of the project himself. After a segment of the bridge collapses, a defeated Saito has Nicholson brought to his office from the oven and explains that if he fails to meet the deadline, he will be forced to commit hara-kari. Unmoved, Nicholson insists that the Geneva Convention be adhered to, after which Saito orders him taken back to the oven. Meanwhile Shears, who was found near death by some friendly villagers, recovers and begins a solitary boat trip down river. Days later, his water exhausted, Shears passes out, leaving his boat to drift aimlessly. Plagued by ineptness and sabotage in his efforts to build the bridge, Saito orders the weakened and dehydrated Nicholson pulled from the oven and brought to his office where he grants a general amnesty to the officers and declares it will not be necessary for them to perform manual labor. As the men cheer Nicholson's victory, Saito sits in his office, broken and sobbing. Upon inspecting the bridge, Nicholson criticizes the workers' cavalier attitude and asks Capt. Reeves, who worked as a civilian engineer, for advice. Reeves observes that the river bottom at the present location is soft mud and suggests moving downstream where the bottom is solid bedrock. When Maj. Hughes, a public works engineer, criticizes the men's lack of teamwork, Nicholson declares that they will rebuild the company's morale by building an exemplary bridge. Nicholson, completely oblivious to the fact that he is about to aid and abet the enemy, presents a plan to Saito increasing the men's daily work quota and suggests that Japanese soldiers should work laying track. Seeing his position crumbling, Saito stoicly says that he has already given the order. Shears, meanwhile, has been picked up by a sea rescue plane and brought to a hospital in Ceylon where he is visited by Maj. Warden, the explosives instructor at a nearby British commando school, who invites him to a meeting at the school. At the meeting, Warden explains that he plans to lead a team into Burma to blow up the bridge and asks Shears to join them. Shears, desirous of returning to civilian life, demurs, confessing that he was merely impersonating an officer and therefore is not qualified for the mission. Warden then informs him that the U.S. Navy already knows about his deception and has authorized his transfer to the British commandoes. Faced with possible imprisonment for impersonating an officer, Shears reluctantly accepts the assignment and retains his rank as Major. At the prison camp, Clifton warns Nicholson that he could be charged with treason for collaborating with the enemy. Obsessed by proving the mettle of his British soldiers, Nicholson turns a deaf ear to the medic's protest. At the commando school meanwhile, Shears joins a team comprised of Warden, Chapman and Lt. Joyce, a young recruit wary of killing. As the four parachute into the jungle, Chapman crashes into a tree and is killed. The others are met by Yai, a native guide who hates the Japanese, and four women bearers. As they begin their trek through the jungle, they receive a radio transmission from headquarters informing them that a special train carrying troops and VIPs is scheduled to inaugurate the bridge on the thirteenth, and ordering them to carry out the demolition on that day. Realizing that he cannot finish the bridge by the deadline, Nicholson matter-of-factly tells Clifton that he has asked the officers to work beside the enlisted men and they have volunteered "to a man." After Clifton's protests, Nicholson then recruits wounded men from the infirmary to perform "light labor." In the jungle, meanwhile, Warden's team is accosted by Japanese soldiers, and in the skirmish Warden is shot in the ankle. Warden subsequently stumbles along on his crippled foot, climbing torturous mountain paths, but when they are just six hours away from the bridge, he declares that the others should continue on without him. Angrily denouncing Warden's self-sacrifice as the histrionics of a British gentleman, Shears orders him hoisted onto a stretcher, after which they all continue on together, reaching the bridge just as Nicholson is nailing up a plaque commemorating the work of the British soldiers. That night, as the prisoners put on a show in celebration of the completion of the bridge, concluding with "God Bless the King," Joyce and Shears, aided by the women, pilot a raft filled with plastic explosives to the bridge while Saito, having been bested by the British, makes preparations for hara-kari. After attaching the explosives to the bridge, Joyce takes cover with the detonator while Shears swims back across the river to await the arrival of the train the next day. At daybreak, the commandoes discover that the water level of the river has dropped, exposing the wires leading to the detonator. When the troops march onto the bridge for the ribbon cutting ceremony, Clipton informs Nicholson he wants no part of the festivities and retreats to a hill to watch. As the train whistle is heard in the distance, Nicholson spots the wires and calls Saito to go with him and investigate. Warden watches in disbelief as Nicholson follows the wire to the detonator, incredulous that a British officer would try to prevent an act of sabotage against the enemy. Sneaking up behind Saito, Joyce stabs him in the back with his knife, then informs Nicholson that they have been sent by the British to blow up the bridge. Crazed, Nicholson attacks Joyce, prompting Shears to scream "kill him" and swim to Joyce's defense. As Warden bombards the bridge with mortar shells, the Japanese open fire, wounding Shears and killing Joyce. When the injured Shears dies at Nicholson's feet, the colonel realizes his folly just as he is wounded by mortar fire and falls onto the detonator, setting off the explosives as the train approaches the bridge. As the bridge collapses, sending the train spilling into the river, Clipton surveys the scene and utters "madness."