Once Upon a Time in America

The film is presented in non-chronological order, from the 1920s to the 1960s, and it is largely told through flashbacks from the viewpoint of one person. The specific scenes and their order varies from version to version. The following section describes the full European cut of the film.
The film begins in medias res with gangsters entering a Chinese puppet theater, looking for a marked man. The proprietors slip into a hidden opium den and warn a man named "Noodles", but he pays no attention. In a flashback, he watches the police remove three disfigured corpses from a street. He successfully kills one of the three thugs that are after him but learns that the thugs have murdered his girlfriend while looking for him and finds that someone else has stolen his money. He leaves the city.
David "Noodles" Aaronson struggles as a street kid in the Jewish ghetto on the Lower East Side of Manhattan,[3] in 1920. He and his friends Patrick "Patsy" Goldberg, Phillip "Cockeye" Stein, and little Dominic commit petty crimes under the supervision of the local boss Bugsy. Planning to rob a drunk at the moment a passing truck hides them from a policeman, they're foiled by the older Max Bercovicz, who jumps off of the truck to rob the man himself. Noodles confronts Max but a crooked policeman steals the watch they were fighting over. By chance, they later find him with a teenage girl; Max's camera enables them to blackmail him and start their own gang independent of Bugsy. The boys establish a suitcase money fund, which they hide in a locker at a train station, giving the key to Fat Moe, a reliable friend who's not part of the operation. Noodles is in love with Fat Moe's sister Deborah, who aspires to be a dancer and actress. One day, Bugsy ambushes the boys and shots Dominic, who dies in Noodles' arms. An enraged Noodles stabs Bugsy to death and also stabs a police officer who tried to intervene, but is knocked unconscious by another officer and arrested. He is sentenced to 12 years in prison.
An adult Noodles is released from jail in 1932 and is reacquainted with his old gang: Max, Patsy, and Cockeye, who are now major players in the bootlegging industry during Prohibition. Noodles reunites with Deborah and tries to rekindle their relationship. Meanwhile, the gang meets Carol during a robbery and she soon becomes Max's girlfriend. The gang prospers financially under prohibition as they engage in bootlegging and also provide muscle for union boss Jimmy Conway O'Donnell. Noodles tries to impress Deborah on an extravagant date, but feels rejected when she states that she is leaving for the West Coast to further her acting career. On their way home, they kiss in a limousine, but upon Deborah's reluctance to go further, Noodles rapes her. After witnessing Deborah leave, he becomes remorseful for what he did.
The gang's financial success is threatened when prohibition is repealed. Max considers an offer to begin working with the teamsters union, but Noodles refuses and leaves. Max runs after him and they go to Florida together. While there, Max suggests robbing the New York Federal Reserve Bank, but Noodles sees it as suicidal. Carol, who also fears for Max's life, convinces Noodles to call the police on his friends for a minor offense that will keep them in jail. Later Max knocks him unconscious for his attitude. After gaining consciousness, Noodles finds out that Max, Patsy, and Cockeye have been killed and is consumed with guilt over making the phone call to police, leading to the scenes which begin the film. The first bus to leave the city is going to Buffalo, and Noodles lives there in hiding under an new identity for the next 35 years.
In 1968, Noodles receives a letter informing him that the cemetery where his friends are buried has been sold and asking him to make arrangements for their reburial. Realizing that someone has deduced his identity, Noodles returns to Manhattan and stays with Fat Moe above his restaurant. The bodies have already been removed to a new mausoleum; while visiting, he discovers the key to the gang's old railway locker and notes the license plate of a car that is following him. Going to the locker, he discovers a suitcase full of cash and a note saying the money is a down payment on his next job.
Noodles visits Carol, who lives at a retirement home run by the Bailey Foundation. She tells him that Max caused the gang's death by opening fire on the police. While at the home, Noodles sees a photo of Deborah at the institution's dedication. Noodles finds the car from the cemetery leaving the lavish estate of Secretary Bailey, the foundation's sponsor and an embattled political figure whose name has been mentioned in news reports. The car explodes, killing a district attorney. Noodles tracks down Deborah, who is now a successful actress. He questions her about Secretary Bailey, telling her that he has received an invitation to a party at Bailey's house. Deborah claims not to know much about Bailey, but Noodles already knows they have lived together for years. In the end Deborah is forced to introduce Noodles to Bailey's son David, who is named after Noodles and who casually appeared there. He resembles the adolescent Max. Knowing the chilling truth that Bailey is Max, Noodles breaks with Deborah and leaves and Deborah cannot look at herself anymore.
At the party at Secretary Bailey's house, Max meets with Jimmy O'Donnell. He is incensed that someone attempted to kill him with a car bomb but folds, signing over his position and interests in exchange for his son's safety. Noodles arrives and the two go to Max's office. Max explains that corrupt policemen helped him betray him, fake his own death, kill his friends and steal the gang's money in order to begin a new life as Mr. Bailey, a man with contacts to the teamsterÂ´s union. Noodles calls him crazy. Now faced with ruin and the specter of a teamster assassination, Max asks Noodles to kill him. Noodles refuses despite Max's permission and goading, because, in his eyes, Max died with the gang. As Noodles leaves Bailey's estate, he hears a garbage truck start up and looks back to see Max standing at the driveway's gated entrance. As he begins to walk towards Noodles, the truck passes between them. The truck passes and Noodles sees its auger grinding down rubbish, the man nowhere to be seen. It is implied Max committed suicide by throwing himself into the truck.
Noodles reflects and the final scene of the film becomes a flashback to the young adult Noodles entering the Chinese puppet theatre seen at the beginning of the movie after the murder of his gang, beginning to smoke opium in the hidden opium den in order to forget the horrible event, and finally smiling far away from reality. It is implied, Noodles finally feels happiness in real life after what happened and therefore also feels a catharsis regarding his past life immersed in suffering he could only cope with the help of the fake happiness he achieved back then. It is also implied, he can from now on move on with his own life without having to look back anymore, which he also does.

Question: Where do the proprietors slip into?
Answer: The proprietors slip into a hidden opium den.