Typically suspenseful movie of the period, with chair gripping aerial sequences and plot twists aplenty.
Concludes happily (despite sad losses) on a humorous note.
First-rate acting.
This looks back to Howard Hawks' pre-code film, The Dawn Patrol, which was about WWI flyers and their response to the seeming inevitability of death. Their survival under pressure forms a bond and an unshakeable code. And it also looks forward to Casablanca whose romance is very similar.
These men deliver the mail in the fog over the Andes. But they will transit anything, including nitroglycerine! Cary Grant is the boss who hires a flyer now married to the only woman he ever cared about... And the new man is reckoned to have once bailed out of a crash which killed his co-pilot... thus breaking the code.
This is Cary Grant's first really successful dramatic role. And it is Rita Hayworth's breakout from support parts in B films. Both are excellent. Two out of three ain't bad; Jean Arthur is badly cast as the showgirl interloper who stumbles on this exotic other world.
There's shadows and fog, and life threatening heroics with plenty of action and a lot of fast, tough crosstalk. There's a gallery of mavericks who turn up on a mountain in South America determined to get the mail out, no matter what. These are the thematic threads that Hawks would weave over his career, creating a kind of genre of his own.