Lawrence is a rich kid with a bad accent and a large debt. After his father refuses to help him out, Lawrence escapes his angry debtors by jumping on a Peace Corp flight to Southeast Asia, where is assigned to build a bridge for the local villagers with American-As-Apple-Pie WSU Grad Tom Tuttle and the beautiful and down-to earth Beth Wexler. What they don't realize is that the bridge is coveted by the U.S. Army, a local Communist force, and a powerful drug lord. Together with the help of At Toon, the only English speaking native, they must fight off the three opposing forces and find out what is right for the villagers, as well as themselves.
- Yale University, 1963
Lawrence Bourne III (Tom Hanks) has a $28,000 gambling debt, and loan shark, Albert Bordonaro (Alan Arbus), gives him one day to come up with the money. Lawrence bets on a football game, and listens to the outcome over a portable radio while he attends his graduation ceremony. When Lawrence's team loses, Albert Bordonaro waits until the day is over to exact revenge. Meanwhile, Lawrence asks his wealthy father, Lawrence Bourne, Jr. (George Plimpton), to loan him the money, but he refuses, insisting that Lawrence needs to learn responsibility. When a thug named Cicero (Ji-Tu Cumbaka) arrives to collect the payment, Lawrence flees to La Guardia airport in New York, as Cicero pursues.
Lawrence runs to meet his roommate, Kent Sutcliffe (Xander Berkeley), who is about to depart for a two-year Peace Corps mission to Thailand. Lawrence begs Kent to let him take his place on the airplane to avoid being beaten or killed by Bordonaro's thug. Kent agrees on condition that Lawrence give him his vintage automobile and his girl friend, Bootsy (Jude Mussetter). Lawrence agrees, and escapes just in time.
On the airplane, Lawrence meets Tom Tuttle (John Candy) from Tacoma, Washington, who talks non-stop. Lawrence soon realizes he made a mistake, and has no interest in social service. However, when he meets a pretty girl on the team named Beth Wexler (Rita Wilson), he changes his mind. Beth believes him to be Kent Suttcliffe, her assigned partner in Thailand. After they talk for hours on the flight, Lawrence makes a pass at Beth, and she is offended that he was trying to seduce her the whole time. After learning that Lawrence took Kent's place, Beth berates him. When he implies that she is "frigid," Beth slaps him.
Arriving in Thailand, Beth is welcomed by John Reynolds (Tim Thomerson), who heads the agricultural department where she has been assigned. Beth is smitten. Lawrence begs to go home, but is sent to a rural village with Beth and Tom Tuttle where they will oversee a bridge-building project. John Reynolds warns them about guerrilla rebels in the area, and a drug lord named Chung Mee (Ernest Harada) who inhabits the surrounding jungle. Lawrence greets the villagers while still wearing his dinner jacket and bow tie, as Communist guerrillas watch the team's arrival through binoculars.
As they settle into their huts, Lawrence makes a half-hearted apology to Beth. Tom Tuttle angers villagers by suggesting they cut down sacred teak forest trees to use for the bridge. However, a young English-speaking villager named At Toon (Gedde Watanabe) explains the misunderstanding, and befriends Lawrence. Sometime later, Tom Tuttle falls into a trap and is held captive by guerrillas in the jungle for much of the duration of the movie.
Lawrence initiates a poker game with the villagers, and ignores Beth when she insists that they search for Tom. Drug lord Chung Mee arrives and demands a seat at the poker table. At Toon tells Lawrence that Chung Mee is a dangerous black market dealer and they should do what he asks. Chung Mee's men bind and blindfold Lawrence and At Toon, and take them to his jungle fortress. Chung Mee asks how quickly they can build the bridge, explaining it will increase his opium business. Lawrence proposes a deal to expedite construction, asking for an airline ticket home and $48,000. Chung Mee agrees, on condition it is completed in six weeks. Beth is surprised to see apathetic Lawrence enthusiastically working with the villagers on the bridge.
Sometime later, John Reynolds returns to the village to check on the team, and Beth is excited by his arrival. She asks for his help finding Tom Tuttle, but John believes Tom is gone for good. When Beth expresses her interest in John, he introduces her to his knife, "Mike," and gives her a phallic Thai sculpture. Her interest begins to wane as she questions his sanity.
Meanwhile, the captive Tom Tuttle is tied to a post by the guerrillas and meets a general named Souvanna (Clyde Kusatsu), who brainwashes the American patriot into becoming a communist rebel. Souvanna assigns Tom to build the bridge and allow no one to interfere. Tom returns to the village, and Beth greets him warmly, and he gets to work with Lawrence and the villagers constructing the bridge.
In time, the bridge is completed, and Lawrence surprises Beth with a romantic date. As they dance, she warms up to him, and invites him into her hut. Lawrence respectfully declines, but kisses her goodnight and expresses his interest in her. When Beth enters her hut, John Reynolds surprises her, disguised in camouflage. He reveals that he works for a covert operation tasked to eliminate communist rebels, and Beth realizes he is insane. Reynolds explains they will use the bridge to bring in a convoy, destroying the village and creating a military outpost to attack the rebels. Beth screams as Reynolds uses chloroform to knock her out. At Toon witnesses Reynolds carrying Beth into the jungle, but does not realize she is unconscious.
The next night, villagers celebrate the completion of the bridge, and Lawrence worries when Beth is missing. When At Toon admits to seeing her leave with John Reynolds, Lawrence is upset that Beth "led him on." Chung Mee's bodyguard, Lucille, arrives with Lawrence's money, and he attempts to seduce the mysterious woman. However, Lucille plans to kill him, but At Toon arrives to stop her. She confesses that Beth and Reynolds are with Chung Mee, and Lawrence realizes Beth was kidnapped. He asks Tom Tuttle for his help, but the brainwashed man refuses to leave his post protecting the bridge.
Lawrence sneaks into Chung Mee's compound with At Toon's help, and finds Beth in a drugged state. He hides when John Reynolds approaches, but when Reynolds prepares to have his way with Beth, Lawrence attacks. He knocks Reynolds unconscious and carries Beth to safety, as Chung Mee's men pursue. Lawrence, Beth, and At Toon escape and return to the village. Lawrence demands they pack up and leave, but Beth insists they blow-up the bridge first. He refuses, and spontaneously professes his love for Beth, who asserts that if he really loved her, he would destroy the bridge. However, she takes back her statement, preferring he destroy the bridge based on his own sense of moral obligation. Lawrence agrees it is the right thing to do, and Beth is pleased.
As Beth shares their plan with the villagers, Tom Tuttle arrives holding a lit stick of dynamite, to prevent them from destroying the bridge. Lawrence overtakes Tom, slapping him repeatedly. Tom returns to his patriotic self, and advises them on the best way to destroy the bridge. They get to work lining the bridge with dynamite, as both Chung Mee and the communist rebels descend upon the village. When the explosives fail to ignite, Lawrence returns to the bridge to fix the problem, and John Reynolds holds him at knife point. Lawrence knocks Reynolds over the side and jumps into the river after him. Tom Tuttle sets off the dynamite, and the bridge blows up.
Afterward, Lawrence floats face down from the river, and Beth pulls him out and gives him mouth-to-mouth resuscitation. He revives and begins kissing her, as villagers cheer. In the final scene some time later, Lawrence writes a letter home, telling his parents that the Peace Corps has changed his life, and shares his plans to open a casino in the village.