This John Wayne adventure is set in South America's rugged Andes Mountains. The Duke has been assigned by a powerful US mining magnate to build a railroad to his newest mines. The two men lock horns over the route the railroad will take. The cost-conscious, people-insensitive industrialist wants to take the shortest route, right through the mountain. But building the tunnel will be extremely dangerous. Wayne wants to do it more safely and build a bridge. Eventually, the engineer is forced to acquiesce with his boss. Later the engineer meets and falls in love with a pretty young woman who turns out to be his hated boss's daughter and this only makes matters worse.