American engineer Johnny Munroe (John Wayne) is hired by South American tycoon Frederick Alexander (Cedric Hardwicke) to bore a tunnel for a new railroad in the Andes Mountains. On the job, he falls for Alexander's beautiful daughter, Maura (Laraine Day). Facing dangerous conditions and serious design challenges, Munroe struggles to complete the job he was hired to do, while simultaneously struggling to convince his skeptical, overprotective employer that he is a worthy suitor for Maura.