They'll always have Paris.
Its plot is laughable: If Czech patriot Victor Laszlo could just get a hold of those letters of transit signed by General DeGaulle, he could travel anywhere and the Nazis couldn't lay a hand on him. And a scene in which an entire nightclub sings "La Marseillaise" in defiance of the jack-booted Nazi thugs who occupy Casablanca would be incredibly hokey in any other movie.
And yet it works.
Humphrey Bogart plays Rick, an embittered American saloon owner sitting out WWII in Casablanca. Then out "of all the gin joints, in all the towns, in all the world," in walks Ilsa Lund, the luminous Ingrid Bergman. Everyone in the cast is remarkable, the bit players as well as the stars, but Casablanca wouldn't be Casablanca without Ingrid Bergman.
Ilsa Lund isn't so much a person as she is a concept. Victor Laszlo cannot continue fighting the Nazis without her. Not because of her courage or her skills as an agitator, but because she is his inspiration. She inspires almost murderous desire in Rick, who wants nothing more than getting her back when they first meet again in Casablanca. So Bergman must be both the ideal helpmate and the ultimate lover. Few women could pull that off, but she does. She positively glows. Take a look at this scene, in which Rick first discovers Ilsa is in Casablanca:
Is it any wonder he's smitten?