And then, "of all the gin joints in all the towns in the world," Rick's old love Elsa Lund (a luminous Ingrid Bergman) walks into Rick's cafe, together with her husband Victor Laslo (Paul Henreid). Laslo is a leader of the resistance and has been in a German concentration camp. He and Elsa need passes to be able to leave Casablanca safely and as it happens, Rick has just such papers secretly in his possession... but he is still angry with Elsa because she left him in the lurch...
This great film gets everything just right. The casting is inspired: Bogart, bruised and disillusioned, but also humane for finally he is roused to action; Bergman, young and more beautiful than ever; but also Veidt (sinister), Rains (frivolous) and Henreid (with a concentration camp scar on his brow) are perfect in their roles, as are the other characters in the smaller parts. In fact, most characters in the film go through a sort of catharsis, realizing what is really important for them in life. They grow into better human beings than they were, and that is what makes this a wonderful film.
The dialogue is spare and cynical, the B/W photography shiny as new, the story is told with economy and most of the emotional effect is created by indirection.
The screenplay was based on a theater play, but worked on day by day by a group of writers and - importantly - Bogart himself when the filming was already in progress. Nobody knew where the story was going (making Bergman really confused, but that exactly fits her role), but in the end the screenplay just got it right, too, including the last words where Captain Renault and Rick walk off to try their luck elsewhere in Africa: "This could be the start of a beautiful friendship."
And for once the Academy Awards also got it right, with Oscars for Best Film, Best Director and Best Screenplay. And then to think that Warner originally intended to make a petty B-film!