After a rough week of railroad tunnel construction in the remote Andes Mountains, hard-working American engineer Johnny Munroe heads for Tenango, the nearest city, to unwind. While brash Johnny enjoys himself into the morning hours, his older partner, Pop Mathews, discusses the tunnel's progress with Frederick Alexander, the American tycoon who is bankrolling the railroad, and Ricky Vegas, Alexander's engineer nephew. Later, after he has been criticized by the austere Alexander, a half-drunk Johnny follows a beautiful young woman into a church, where Sunday Mass is being celebrated. Although gently rebuffed at the church, Johnny later pursues the woman to her home, and there learns that she is Maura Alexander, his employer's beloved daughter and Ricky's cousin. Following the strict local customs of courtship, Maura, who is equally taken with Johnny, invites him in for a ten-minute chat. When Alexander sees Johnny innocently touching Maura, he throws the engineer out of his home and chastises Maura for associating with a "drunken lout." Maura rejects her father's opinion of Johnny and warns him that she is no longer a child he can control. The next day, Pop informs Johnny that Alexander has turned down their request to add a concrete lining to the tunnel as a safety precaution, and a furious Johnny rushes to Tenango and accuses Alexander of putting his men in jeopardy. Alexander is unmoved by Johnny's tirade and further inflames the situation by making most of the local timber and dynamite unavailable to him. Alexander then sequesters Maura at his country estate and makes plans to take her to Europe. In defiance of her father, Maura follows the advice of her sympathetic tutor, Miss Ellen Braithwaite, and rides her horse toward Tenango, where Johnny is waiting to meet her. Before she arrives, however, Johnny learns that she has been sent to the country and starts to drive there. Johnny and Maura meet along the road, but while Johnny heads back to town, his jeep runs out of gas. As the day ends, Maura and Johnny try to find their way on foot but become stranded among some Incan ruins. During the night, Alexander and a group of men who have been searching for Maura spot their fire and come upon the couple as they huddle together. Caught in this seemingly compromising situation, Maura and Johnny marry quickly, and Maura moves into Johnny's construction camp home. Johnny is immediately called to the site of a tunnel accident, in which Pop and Ricky are nearly killed. Although Johnny reassures Maura that the tunnel will be finished in three months, Ricky insists that the project will take at least a year. Three months later, the tunnel is still incomplete, and Maura's faith in Johnny begins to crumble. Then, after yet another devastating tunnel collapse, the neglected Maura decides to leave Johnny and return to her father. Devastated by Maura's departure, Johnny becomes obsessed with completing the railroad, but demands that the mountain tunnel be replaced by a river bridge. Alexander reluctantly agrees to Johnny's ambitious plan, in which he promises to finish the bridge with the remainder of the tunnel budget. Johnny's determination to meet his self-imposed deadline soon drives his best crew members to quit and Pop to end their partnership. When confronted by a remorseful Maura, Johnny coldly tells her that he is working only for money now and will continue until he earns enough to "buy" her back from her father. As Alexander had earlier predicted, however, the bridge's future is threatened by the arrival of the rainy season. Before the middle section of the bridge has been inserted, an intense rain storm hits, causing the river water to rise upstream and gather force as it rolls toward the bridge. With the rain waters only hours away, Johnny tries to complete the bridge, but finally gives up when the section becomes stuck and his disgruntled crew quits. Johnny's defeat is soon reversed after Maura and Pop return to camp, and the crew agrees to finish the job. As the wall of water approaches the bridge, Johnny risks his life to drive some train cars onto the still unfinished section to provide ballast. Although the middle section collapses and tumbles into the raging river along with the train, the bridge's foundations remain secure. Later, Alexander admits to a change of heart concerning Johnny and, after giving him and Maura his blessing, leaves for Connecticut with Miss Braithwaite. Maura then insists that Johnny abandon his bridge until they have returned from their long-awaited honeymoon.