On the eve of Prohibition in 1920, Atlantic City county treasurer Enoch "Nucky" Thompson is looking to cash in. Prohibition may have outlawed alcohol, but in Nucky's world it has opened up highly lucrative opportunities for gangsters and rumrunners looking to provide "liquid gold" for a thirsty nation. In a city defined by notorious backroom politics and vicious power struggles, Nucky battles a relentless federal agent, ambitious underlings, and opportunistic rivals—including Arnold Rothstein, Lucky Luciano, and Al Capone.

It's 1921 and after rigging the Atlantic City mayoral election, Nucky appears to be in control of the city. However, Nucky's grip soon begins to unravel as he is arrested for election fraud and members of both his inner circle and family begin to question their allegiances, while mobsters including Arnold Rothstein, Lucky Luciano, Meyer Lansky, and Al Capone wait in the wings, looking for the right opportunity to grab a bigger piece of Nucky's pie.

Atlantic City, 1922: The Roaring '20s are about to begin in earnest and despite a booming economy, alcohol is scarce and gangster violence is heating up. Amidst this backdrop, Nucky, whose marriage to Margaret has become a sham after she signed away his highway windfall to the church, faces the challenge of mending old relationships while encountering new competition from hair-trigger gangster Gyp Rosetti, who builds a strategic bulkhead between New York and Atlantic City in an effort to siphon off Nucky's alcohol business.

Atlantic City, 1924: Nucky is laying low at the end of the Boardwalk after barely surviving an overthrow by gangster Gyp Rosetti. But the calm will be short-lived, as Nucky faces new challenges, including a clash with a violent Harlem power broker, a battle with his brother Eli over Eli's college-age son, and the irresistible lure of lucrative—and perilous—opportunities in Florida.

It's 1931 and in stark contrast with the Roaring '20s, the country is now in the throes of the Great Depression. With the end of Prohibition in sight, Nucky looks to legitimize himself by forging alliances with liquor producers.