In early 1943, World War II British prisoners arrive by train at a Japanese prison camp in Burma. The commandant, Colonel Saito (Sessue Hayakawa), informs them that all prisoners, regardless of rank, are to work on the construction of a railway bridge over the River Kwai that will connect Bangkok and Rangoon. The senior British officer, Lieutenant Colonel Nicholson (Alec Guinness), reminds Saito that the Geneva Conventions exempt officers from manual labour.
At the following morning's assembly, Nicholson orders his officers to remain behind when the enlisted men are sent off to work. Saito slaps him across the face with his copy of the conventions and threatens to have them shot, but Nicholson refuses to back down. When Major Clipton (James Donald), the British medical officer, intervenes, telling Saito there are too many witnesses for him to get away with murdering the officers, Saito leaves the officers standing all day in the intense tropical heat. That evening, the officers are placed in a punishment hut, while Nicholson is locked in an iron box.
Meanwhile, three prisoners attempt to escape. Two are shot dead, but United States Navy Commander Shears (William Holden), gets away, although badly wounded. He stumbles into a village of natives who help him leave by boat.
Nicholson refuses to compromise. Meanwhile, the prisoners are working as little as possible and sabotaging whatever they can. Should Saito fail to meet his deadline, he would be obliged to commit ritual suicide. Desperate, Saito uses the anniversary of Japan's victory in the Russo-Japanese War as an excuse to save face and announces a general amnesty, releasing Nicholson and his officers from manual labor.
Nicholson conducts an inspection and is shocked by the poor job being done by his men. Over the protests of some of his officers, he allows Captain Reeves (Peter Williams) and Major Hughes (John Boxer) to design and build a proper bridge, despite its military value to the Japanese, for the sake of maintaining his men's morale. The Japanese engineers had chosen a poor site, so the original construction is abandoned and a new bridge is begun downstream.
Shears is enjoying his hospital stay in Ceylon with a beautiful nurse (Ann Sears), when British Major Warden (Jack Hawkins) informs him that the U.S. Navy has transferred him over to the British to join a commando mission to destroy the bridge before it's completed. Shears is appalled at the idea of returning to a place from which he nearly died during escape. He confesses he is not an officer, but merely had appropriated an officer's uniform prior to his capture, expecting that this revelation will invalidate the transfer order. However, Warden responds he already knew the truth and tells Shears that the American Navy's desire to avoid dealing with the embarrassment of his actions is the very reason they agreed to his transfer. Assured that he will be allowed to retain the privileges of being an officer and accepting that he actually has no choice, Shears relents and "volunteers" for the mission. The commando team consists of four men.
Meanwhile, Nicholson drives his men hard to complete the bridge on time. For him, its completion will exemplify the ingenuity and hard work of the British Army for generations, long after the war's end. When he asks that their Japanese counterparts join in as well, a resigned Saito replies that he has already given the order.
The commandos parachute in, with one man killed on landing, leaving three to complete the mission. Later, Warden is wounded in an encounter with a Japanese patrol and has to be carried on a litter. He, Shears, and Canadian Lieutenant Joyce (Geoffrey Horne) reach the river in time with the assistance of Siamese women bearers and their village chief, Khun Yai. Under cover of darkness, Shears and Joyce plant explosives on the bridge towers below the water line.
A train carrying soldiers and important dignitaries is scheduled to be the first to cross the bridge the following day, so Warden waits to destroy both. However, at daybreak the commandos are horrified to see that the water level has dropped, exposing the wire connecting the explosives to the detonator. Making a final inspection, Nicholson spots the wire and brings it to Saito's attention. As the train is heard approaching, they hurry down to the riverbank to investigate. The commandos are shocked that their own man is about to uncover the plot.
Joyce, manning the detonator, breaks cover and stabs Saito to death. Aghast, Nicholson yells for help, while attempting to stop Joyce from reaching the detonator. As he wrestles with Nicholson, Joyce tells Nicholson that he is a British officer under orders to destroy the bridge. When Joyce is shot dead by Japanese fire, Shears swims across the river, but is fatally wounded as he reaches Nicholson. Recognizing the dying Shears, Nicholson exclaims, "What have I done?" Warden fires his mortar, mortally wounding Nicholson. The dazed colonel stumbles towards the detonator and collapses on the plunger, just in time to blow up the bridge and send the train hurtling into the river below. Witnessing the carnage, Clipton shakes his head muttering, "Madness!Â ... Madness!"
Who is working as little as possible and sabotaging whatever they can?
The prisoners