Set in southeast Asia during World War II, The Bridge on the River Kwai is about a clash of cultures, partly between East and West, the Orient and the Occident, but even more so between America and Great Britain, between American cynicism, individualism, and egalitarianism on the one hand, and idealistic, class-conscious British collectivism on the other.
Shears (William Holden) is the sole American in a Japanese prisoner-of-war camp, while the rest of the prisoners are British. This underscores his individualism. It turns out later that he is really an enlisted man posing as an officer, showing his contempt for class distinctions. He thought being an officer would mean that he would not have to work as hard as a prisoner. Since that did not go as planned, he bribes the guards to give him light duty. And he regularly ridicules the British dedication to the war effort.
The British, on the other hand, regard the distinction between officers and enlisted men as sacrosanct. This is especially embodied in commanding officer Colonel Nicholson (Alec Guinness), who balks when he finds out, as did Shears, that the Japanese camp commander, Colonel Saito (Sessue Hayakawa), requires officers to work right alongside the enlisted men. He refuses to order his men to work and suffers several days of harsh punishment as a result. Saito eventually has to relent and let the British officers merely supervise the work of enlisted men, because he needs to get the title bridge built.
But then, halfway through the movie, everything goes into reverse, and the distinction between American and British attitudes begins to collapse. Shears escapes from the camp and winds up in a British hospital, where he pushes his luck by continuing to pretend that he is an officer in order to get better treatment. Major Warden (Jack Hawkins), a British officer, finds out about this and coerces Shears into agreeing to go back into the jungle with him so they can sabotage the bridge, which will allow Shears to avoid being prosecuted for impersonating an officer. The other member of the team will be Lieutenant Joyce (Geoffrey Horne), so Warden says he will make Shears a major for the purpose of the mission, so that the rigid distinction between officers and enlisted men will not have to be observed.
Meanwhile, back in the jungle, Nicholson is anxious to get the bridge built, and to build it as an example of British engineering excellence. The other officers are in favor of surreptitiously delaying the building of the bridge and making sure that it is inferior, so as to minimize their assistance to the enemy, but Nicholson thinks that building a bridge that will redound to British glory for hundreds of years is more important than its effect on the war. Furthermore, when he realizes that they are behind schedule, he violates the very code he fought for, and gets the officers to work alongside the enlisted men. He even asks men in the camp hospital to get out of their beds and pitch in.
In spite of himself, Shears ends up being the officer in charge of the mission, sacrificing himself in order to destroy the bridge, while Nicholson dies realizing the enormity of what he has done.