In the early years of World War II, the Moroccan city of Casablanca attracts people from all over. Many are transients trying to get out of Europe; a few are just trying to make a buck. Most of them -- gamblers and refugees, Nazis, resistance fighters, and plain old crooks -- find their way to Rick's Café Américain, a swank nightclub owned by American expatriate Rick Blaine (Humphrey Bogart). Though we learn later that Rick once harbored enough idealism to put himself at risk to fight facism, he's now embittered and cynical, professing to be neutral and detached: "I stick my neck out for nobody."

Ugarte (Peter Lorre) comes to Rick's with letters of transit he obtained by killing two German couriers. The papers allow the bearer to travel freely around German-controlled Europe, including to neutral Lisbon, Portugal; from Lisbon, it's relatively easy to get to the United States. They are almost priceless to any of the refugees stranded in Casablanca. Ugarte plans to make his fortune by selling them to the highest bidder, who is due to arrive at the club later that night. However, before the exchange can take place, Ugarte is arrested by the police under the command of Captain Louis Renault (Claude Rains). A corrupt Vichy official, Renault accommodates the Nazis. Unbeknownst to Renault and the Nazis, Ugarte had left the letters with Rick for safekeeping, because "...somehow, just because you despise me, you are the only one I trust."

Now the reason for Rick's bitterness re-enters his life. Ilsa Lund (Ingrid Bergman) arrives with her husband Victor Laszlo (Paul Henreid) to purchase the letters. Laszlo is a renowned Czech Resistance leader who has escaped from a Nazi concentration camp. They must have the letters to escape to America to continue his work. At the time Ilsa first met and fell in love with Rick in Paris, she believed her husband had been killed. When she discovered that he was still alive, she left Rick abruptly without explanation and returned to Laszlo, leaving Rick feeling betrayed. After the club closes, Ilsa returns to try to explain, but Rick is drunk and bitterly refuses to listen.

The next night, Laszlo, suspecting that Rick has the letters, speaks with him privately about obtaining them. They're interrupted when a group of Nazi officers, led by Major Strasser (Conrad Veidt), begins to sing "Die Wacht am Rhein," a German patriotic song. Infuriated, Laszlo orders the house band to play "La Marseillaise." The band leader looks to Rick for guidance; he nods. Laszlo starts singing, alone at first, then long-suppressed patriotic fervor grips the crowd and everyone joins in, drowning out the Germans. In retaliation, Strasser orders Renault to close the club.

At different times Rick and Ilsa torment themselves by asking the club's piano player, Sam (Arthur "Dooley" Wilson), to play "As Time Goes By," a song they loved when they were together in Paris. The famous line "Play it again, Sam," which refers to this song, doesn't ac