In 1943, British war prisoners in Siam wer employed by the Japanese to build a railroad from Bangkok to Rangoon. A specific battalion under the command of Colonel Nicholson is ordered to build a railway bridge of the River Kwai and thus establish a vital link in Japanese communications. A battle of ethics develops between Nicholson and the Japanese commandant, Colonel Saito, when the British officers are ordered to work alongside the men. Citing the Geneva Convention, Nicholson refuses to allow it and is tortured through brutal imprisonment. He refuses to yield and wins both a moral victory and the respect of the commandant.

Nicholson then takes charge of the building operations, intending to make the bridge a tribute to British know-how and resourcefulness. He hopes that this will restore his men's morale. Nicholson becomes so obsessed with the bridge as a symbol of British invincibility that he drives his men ruthlessly, losing sight of the fact that they are now collaborating with the enemy.

As the work nears completion, a small group of Allied commandoes, led by the British Major Warden and an American sailor named Shears, are parachuted into the jungle. Their orders are to destroy the bridge as soon as it has been finished, before an expected train of Japanese troop reinforcement can cross the river. After nailing up a commemorative plaque in honor of the soldiers' work, Nicholson notices the explosives wired to the bridge. Suddenly crazed, he alerts the Japanese commandant. Nicholson realizes too late that he has exposed the commandoes. In the ensuing struggle for the bridge, the commandant, colonel, and several of the commandoes are killed. As he is dying, Nicholson falls on the plunger, exploding the bridge and the trainload of Japanese soldiers. Aghast at the scene, the horrified British Medical Officer shouts, "Madness! Madness!"
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Synopsis written by Louis Castelli, Ph.D from his book, David Lean: References and Resources.