Johnny Munroe (John Wayne) and his partner, Mathews (James Gleason), are hired by American industrialist Frederick Alexander (Sir Cedric Hardwicke) to construct a railway from his mines in the Andes to the coast. Johnny favors building a bridge over a river, but Alexander wants a shorter route through a tunnel. Tensions rise between the two as Johnny becomes interested in Alexander's daughter, Maura (Laraine Day). When Alexander cuts off supplies to sabotage the project, Johnny dynamites the tunnel and builds the bridge despite challenges and pushback from his crew. During a storm, Maura arrives with the old crew to help save the bridge and Johnny and Maura are reunited. Meanwhile, Alexander finds love with Maura's duenna and goes on a honeymoon.