In South America, the daunting mountains and dangerous weather have hampered the operations of Trans-Andean European Air Mail, a 1930s-era airline. Charged with delivering a serum to stem an outbreak of Poliomyelitis in Rio de Janeiro, Auguste Pellerin (Robert Montgomery (actor)) conquers his fears, but is reprimanded by the airline's stern director, A. Riviére (John Barrymore) for coming in late.