All time great movie: Casablanca

Casablanca is a movie that was released in the middle of the second world war (in December 1942), and was set in the Vichy controlled city of Casablanca in Morocco. For those unaware, the Vichy regime was the one that took over France after the Germans defeated the French army in WWII and collaborated with the Nazis. They also took over French controlled regions in North Africa.
The movie made a good amount of money when it was released, but was not seen as an incredible movie at the time of release. However, as time passed, the movie made more money, and was incredibly acclaimed as one of the all-time greats. Nowadays, it would almost always make it in the top 10 movies of all times. It won 3 oscars at the time it was released, but none for its star cast.
The movie starts the principal characters of Humphrey Bogart (Rick Blaine) and Ingrid Bergman (Ilsa Lund). Rick runs a popular cafe called 'Rick's Cafe Americain' in Casablance, and has a mix of Nazi officials, refugees and thieves as patrons. These are times of war, and hence there is a fair amount of tension in the air. Rick is a cynic, but professes disliking for the Nazis.
Rick is a bitter man, and the reason for his bitterness is Ilsa. He and Ilsa were in love, when Ilsa believed her husband had died in German captivity, but when she found he was alive, she left Rick abruptly, causing him to become bitter.
Rick has obtained 2 letters that very important because they allow their bearers to travel freely around German territory, and also to Portugal which is neutral territory, and from there the bearer could escape to the US. There is another cynical character in the story, Captain Louis Renault, who is a Vichy official and provides support for the Nazis. At this time, Ilsa re-enters his life, arriving with her husband Victor Laszlo. Victor is a resistance leader, being searched by the Nazis and must escape to the US. Those papers are of vital importance to Ilsa and Laszlo as they would allow him to escape.
Ilsa and Laszlo suspect that Rick has these papers and are going to speak to him about these papers, when there is a confrontation in the cafe. German officers start singing the German patriotic song, 'Die Wacht am Rhein', and the others in the case, start retaliating by singing the international socialist song, 'La Marseillaise'. Rick encourages the cafe band to also sing countering the Germans, and when the German voices are drowned out, the Germans are angry, and the German major orders Renault to close the cafe.
In the night, Ilsa threatens Rick with a gun unless he gives her the papers, but he refuses. She is unable to shoot him as she still loves him. Rick changes his mind and decides to help Laszlo. In the meantime, the German major Strasser arranges to arrest Laszlo, but Rick tricks Renault to release him, and then threatens him with a gun to let Laszlo escape. Rick also pushes Ilsa to leave with Laszlo (actually, there was no other alternative since the Hollywood code of conduct prevents a wife from going with anybody other than her husband).
In the finale, Rick shoots Strasser, and Renault saves his life. They both decide to leave Casablanca, and you get the idea that these 2 cynics are now part of the resistance. The film has spawned 3 of the most famous lines, 'Louis, I think this is the beginning of a beautiful friendship', 'Here's looking at you, kid', and 'Play it again, Sam' (never actually spoken in the movie).