January 1920. On the eve of Prohibition, Atlantic City's Treasurer, Nucky Thompson, condemns alcohol at a Women's Temperance League meeting, where he is noticed by Margaret Schroeder, a pretty, pregnant housewife who comes to him for help in getting her abusive husband Hans a job. Later that evening, the duplicitous Nucky 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 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, meanwhile, has higher aspirations and ends up making an alliance that could have dire consequences for both him and Nucky.

Nucky deals with the fallout from Jimmy's rogue heist, sending Eli to sow the seeds of doubt in the mind of Margaret, who seeks help in the wake of her husband's demise.

An unlikely witness to the woods massacre threatens to expose the perpetrators, sending both Eli and Van Alden rushing to the hospital. Margaret starts a new job at an upscale French dress shop at the Ritz, while Chalky's team pays a heavy price for taking over Mickey Doyle's bootleg operation.

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 political relationships with ambitious U.S. Senator Walter Edge and Jersey City Mayor Frank Hague, while Eli and Chalky go to extremes to interrogate a local KKK leader.

On the day of the annual St. Patrick's Eve dinner, Nucky deals with Eli's oratory ambitions, as well as a group of disenchanted "Leprechauns". Feeling ignored by Nucky, a frustrated Margaret feeds Van Alden information. 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 accepts an offer from Nucky to change her circumstances. In Chicago, Al's frustration over family issues spills over into his relationship with Jimmy, who scores points with Johnny Torrio for a bold power play involving Sheridan and the neighborhood of Greektown.

After his father Ethan is injured in a fall, Nucky uses the opportunity to refurbish his childhood home. On a double-date with new friend Annabelle and her beaux, Margaret tries to get Nucky to open up about some painful childhood memories. During a visit to an army hospital, Jimmy befriends disfigured vet Richard Harrow, whose sharp eye proves invaluable. Van Alden gets a lead on the woods massacre.

Nucky visits Chicago for the Republican National Convention, where he finds himself intrigued by the candidacy of Warren G. Harding over more established candidates. The D'Alessio gang moves in on Nucky's territory at the expense of Eli, who has been watching over his brother's affairs. Margaret finds herself entangled in Nucky's business affairs, while Van Alden struggles with his wife's desire for a child. Rothstein prepares for legal trouble over his role in fixing the 1919 World Series. Nucky asks Jimmy to return to Atlantic City to reinforce his position.

Nucky asks for Margaret's assistance in backing his mayoral candidate with the passage of women's right to vote, leaving her conflicted about her role. Doyle switches sides back from Rothstein, which leads Nucky to conspire with Chalky against Lansky and the D'Alessios. Angela witnesses Jimmy's violent side against her photographer friend, and considers leaving Atlantic City for good in the aftermath. Van Alden grapples with his emotions, and has encounters with both Margaret and Lucy.

Nucky faces Election Day in Atlantic City and pulls out all the stops to keep power in Republican hands. Torrio brokers a discussion between Nucky and Rothstein to resolve their growing gang war, as well as Rothstein's looming federal indictment over the World Series. Both Jimmy and Eli voice their growing resentment towards Nucky, and find they have an ally in the Commodore. Van Alden seeks a sign on whether or not to leave Atlantic City, but receives unexpected news from Lucy. Margaret weighs new information about Nucky's first marriage and her uncertain future to make a final decision on her role in his life.

Disturbed by a recent nightmare and Billie’s absence, Nucky reluctantly joins Margaret to receive a high honor from the Church. Meanwhile, Gyp makes a deal with Nucky for a onetime liquor shipment, though the mobster’s longtime intentions remain very much up in the air. Van Alden regrets trying to socialize with his co-workers; Margaret takes her women’s-clinic pitch to a higher authority; Lucky and Meyer (Anatol Yusef) protect their investments; Richard sets the record straight with Mickey, and with Nucky.

Eli hopes to get back in Nucky's good graces; Gillian tries to improve her future; Gyp asks for a blessing in New York.