Series Premiere. January, 1920. On the eve of Prohibition, Atlantic City's duplicitous Treasurer, Nucky Thompson, condemns alcohol at a Women's Temperance League meeting, then privately tells his Ward Bosses about the opportunity to make huge profits selling bootleg liquor. At a countdown to midnight blast at Babette's Supper Club, he flirts with Margaret Schroeder, a pretty, pregnant wife looking for help getting her abusive husband Hans a job, and assures Jimmy Darmody, a recently returned WWI vet, that his appointment as "Man Friday" to the new Chief Clerk of the Fourth Ward, Paddy Ryan, will lead to bigger things. Jimmy, who has higher aspirations, ends up befriending Al Capone, a young Chicago thug in town with boss Arnold Rothstein, who's setting up bootleg deals with Nucky. Privy to inside information about a truck shipment leaving town that night, the pair engineers a rogue heist that turns deadly--sending Nucky looking for scapegoats to take the blame.

Investigating a crime which he feels has been pinned on a scapegoat, straight-arrow Agent Nelson Van Alden pays a visit to Nucky and leaves convinced that the Treasurer is "as corrupt as the day is long". Nucky quickly does damage control, enlisting his brother, Sheriff Elias Thompson, to close ranks with their underlings. Nucky discusses the upcoming election with his aging mentor, Commodore Louis Kaestner, with whom he debates the women's vote issue. Later, before heading out for a night on the town, Nucky rebukes an irate Arnold Rothstein over the phone, then meets privately with Margaret Schroeder, who asks him for help in providing for her children. As a long day ends, a traveling salesman named George Baxter, in town for a few days with an unwilling young beauty named Claudia, makes a startling discovery while on the road home to Baltimore.

An unlikely witness threatens to expose the perpetrators of the woods massacre, sending Eli--and Agent Nelson Van Alden--rushing to the hospital. As Nucky re-evaluates his relationship with Lucy, Margaret starts a new job at an upscale French dress shop at the Ritz. Chalky's team pays a heavy price for taking over Mickey Doyle's bootleg operation, causing Chalky to reevaluate his relationship with Nucky. After questioning Angela's loyalty, Jimmy heads west.

In Chicago, Jimmy bonds with one of Johnny Torrio's prostitutes, Pearl, and finds himself increasingly at odds with the hot-headed Al Capone. Back in Atlantic City, Nucky looks to leverage his relationships with ambitious U.S. Senator Walter Edge as well as Jersey City Mayor Frank Hague. Eli and Chalky go to extremes to interrogate a KKK member; Lucky Luciano sets his sights on Jimmy's 'wife'; Margaret impresses at Nucky's lavish birthday party, while Lucy tries to protect her territory.

On the day of the annual St. Patrick's Eve dinner, Nucky deals with his jealous brother's oratory ambitions, as well as a group of disenchanted "Leprechauns." Feeling ignored by Nucky, a frustrated Margaret feeds Van Alden information regarding the whereabouts of a garage filled with barrels of beer. In New York, Arnold Rothstein worries about his involvement in the fixed World Series, while in Chicago, Jimmy copes with the aftermath of a vicious attack orchestrated by Torrio rival Charlie Sheridan.

After the brazen robbery of one of his Ward Bosses, Nucky turns up the heat on Lucky Luciano. An emboldened Margaret stands up to Lucy at the dress shop, and then accepts an offer from Nucky to change her circumstances. In Chicago, Al Capone's frustration over family issues spills over into his relationship with Jimmy, who scores points with Johnny Torrio for a bold power play involving mob heavy Charlie Sheridan and the neighborhood of Greektown.