A standoff occurs when Nicholson adamantly refuses to make his officers do manual labor. The battalion is marched off to work with the officers left standing under the gaze of a machine-gun. Because of Nicholson's unwillingness to back down, he and his officers are placed in the "ovens"- small, galvanized iron boxes sitting in the heat of day. Time passes with Nicholson sweating his life away, while the enlisted men labor away at the bridge. Shears, along with two British soldiers, try to escape but only Shears manages to get away, having been thought drowned in the river. Eventually, Saito reveals why he is desperate to finish the bridge on time. If the bridge is not completed on the pre-set date, Saito will be disgraced and, according to the Japanese code of honor, must kill himself.

Nicholson mentions that he does have men skilled in this type of task but Saito simply locks him back in the "oven." Eventually Saito gives Nicholson and his officers amnesty and Nicholson sees this as a small but well needed victory. Nicholson suggests to his officers that, instead sabotaging the work, why not build the bridge to the best of their ability. Nicholson and his men set to work.

Shears, who survived the escape and is now recovering in a hospital in Ceylon, is asked by Maj. Warden (Jack Hawkins) to come along on a mission to destroy the bridge. Fearing recapture, Shears reveals that he is not even an officer but has been posing as one to get better treatment. Unfortunately for him, Warden already knows and literally blackmails Shears into going on the expedition. After recruiting a fourth man, the team parachutes into the jungle but only Warden, Joyce (Geoffrey Horne), and Shears make the jump safely. After several days of hardship, the team learns of a train scheduled to travel the railroad on the 13th. They choose this day as the day to destroy the Bridge on the River Kwai.

Nicholson informs Doctor Clipton that the bridge won’t be completed on time. To increase manpower, Nicholson recruits wounded men from the 'hospital' to do light labor at the bridge. Far away from the bridge, the team of Shears, Joyce, Warden, and their native porters are resting when they are discovered by a Japanese patrol. A gun battle ensues and Warden and Joyce chase down one of the enemy soldiers. They find and kill him, but Warden is wounded in the struggle. The team presses on, but Warden has difficulty walking on his wounded foot. Finally Warden tells the team to go with out him but Shears refuses, telling Warden what he thinks of him. Warden is carried from then on in a makeshift stretcher. The team finally makes it to the bridge the evening before the train is due.

As Nicholson dedicates a plaque to the bridge, the demolition team watches and makes preparation for its destruction. Shears and Joyce are sent upstream to prepare to float themselves and the equipment down river to the bridge, while Warden stays behind to prepare a mortar assault. Nicholson admires his bridge and reminisces with Saito about his military career. When night falls, Shears, Yai and Joyce make their way to the bridge. Under the clattering boots of sentries, they set charges on the bridge and continue down stream. Shears leaves Joyce with the detonator and crosses to the other side of the river with Yai to cover him. Back in the POW camp, Nicholson’s men hold some entertainment. Nicholson congratulates his men on a job well done.

In the morning, the team discovers that the water level of the river has dropped, making the wires visible. Joyce feverishly tries to cover the cable but it is still spotted by Nicholson. Saito and he follow the wire to Joyce’s location. While their backs are turned, Joyce kills Saito with a blow to his back. Nicholson attempts to stop Joyce and Joyce winds up being shot. Warden begins to mortar the bridge as Shears attempts to cross the river to get to Joyce but is taken down by Japanese soldiers. Nicholson realizes what he has done and tries to get to the detonator but is wounded by a mortar shell. Nicholson stands and makes his way over to the detonator. Keeling over from shell shock, he falls on the detonator just as the train crosses.

Clipton, who had been watching the whole ordeal from a hill, sees the wreckage of the bridge and the bodies of Nicholson, Shears, and Joyce. His only remark to this is "Madness".