By the mid-1950s, Chicago mobsters Paul Ricca, who was recently released from prison, and Tony Accardo have taken the reins of the criminal empire built by their mentor Al Capone. Thanks to Sam Giancana's takeover of the policy racket, the gangsters are now making millions.
With so much money coming in, Accardo and Ricca know they need a place to hide it. They find their answer in a growing city where gambling is now legal: Las Vegas. However, the New York mafia is already there, led by criminal mastermind Meyer Lansky.
Accardo and Ricca meet with Lansky and agree to a partnership that will allow each to profit off the other's casinos. To pay for the construction of new casinos, Accardo and Ricca secure legitimate loans from local banks. They use their legal profits to pay off the loans and then funnel their illegal money through the casinos.
As the operation grows, Accardo and Ricca ask Giancana to run the operation on the ground. Under Giancana's leadership, the Outfit's presence in Las Vegas explodes as gangsters from Chicago move west.
In 1955, while making his rounds in Las Vegas, Giancana spots Willy Bioff, the same man who ratted out Ricca and Nitti 10 years ago and then disappeared. With Ricca's blessing, Giancana kills Bioff with a car bomb.
Over the next two years, Chicago and New York launch an ambitious expansion of Las Vegas, transforming four miles of desert into what will become known as the Las Vegas Strip. As the money pours in, the Outfit comes up with a way to make even more: it's known as the skim, in which money is taken off the tables before it's recorded.
Just as the Outfit's Vegas operations are taking off, an incident in New York City threatens to destroy everything the mob has built. On May 2, 1957, New York mob boss Frank Costello is shot. As the gangster recovers, the police discover a piece of paper in his possession suggesting there was money coming out of the Tropicana, a Las Vegas hotel and casino run by the New York mob.
News of the cops' discovery quickly spreads, exposing the mafia's ties to Las Vegas. After the discovery, banks stop loaning money to buy and expand casinos.
No longer able to secure financing through banks, Accardo turns to Jimmy Hoffa. Thanks to the Outfit's influence, Hoffa is now the president of the Teamsters union and Accardo wants to call in a favor. He convinces Hoffa to loan him money from the Teamsters' pension funds at 3% interest.
As the Outfit's empire continues to grow, Accardo and Ricca protect themselves by naming Giancana the new acting boss, provided that he stay in line. In one move, Accardo and Ricca further distance themselves from the public eye but retain the final say in all big decisions.
For Giancana, taking day-to-day control of the Outfit is a lifelong dream. What he doesn't realize is that he's about to become the target of an ambitious young attorney: Robert F. Kennedy.
To further his career, Kennedy takes a position as the lead investigative counsel to the U.S. Senate, where he goes on a crusade against corrupt labor unions. During the investigation, Kennedy uncovers a link between Hoffa and organized crime and decides to use the power of his panel to investigate the mob.
One of the first men that Kennedy wants to call to the stand is the Outfit's day-to-day boss, Giancana. When Giancana gets word that he's going to be subpoenaed, he skips town and evades federal agents for almost a year. Despite his bosses' orders to keep a low profile, Giancana taunts Kennedy in an interview with a Chicago Tribune reporter.
When federal agents finally catch up with Giancana, Accardo orders Giancana to plead the Fifth Amendment in front of the Senate committee.
On June 9, 1959, Giancana arrives in Washington D.C. ready to go head-to-head with one of the most feared interrogators in the country. As the interrogation proceeds, Giancana repeatedly pleads the Fifth and laughs in Kennedy's face. Accardo later reprimands Giancana for his reckless behavior and sees Giancana as a liability who now risks bringing the whole Chicago mob down.
After Giancana makes a mockery of Kennedy's hearings, Kennedy arranges for the Nevada Gaming Control Board to put Giancana's name on a list of criminals who are barred from entering casinos. Accardo orders Giancana to fix the situation.
In the midst of Accardo and Giancana's growing feud, Ricca is convicted of tax evasion and is once again sent to prison. With Accardo and Giancana's relationship deteriorating and Ricca in prison, the Chicago Outfit is on the verge of a total collapse.