Rick meets Ilsa in Paris. She never tells him she was married and that (she thinks) her husband died in a concentration camp, so he has no clue. In fact, they really don't know much of anything about one another. However, Ilsa unexpectedly leaves Rick, leading him to become the cynical and emotionally shut-down owner of a successful nightclub in Casablanca, Morocco. There, they meet again, and he finds her with a man who turns out to be Victor Laszlo, a leader of the Czech Resistance.

Source: Shmoop