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.

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.

Nucky is forced to readjust his political alliances with a Democratic mayoral candidate promising to wipe out corruption. Jimmy returns from Chicago to deal with the D'Alessio gang, which has allied with Rothstein to import European scotch through Atlantic City, but is arrested by Agent Van Alden for the massacre in the woods. Margaret intercedes with Nucky on Madame Jeunet's behalf, while Angela finds her romantic and artistic ambitions halted by Jimmy's return.

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.

Rothstein vents to Nucky and delivers a message to Gyp. Gaston Means sees an opportunity. Billie gets a new co-start. Van Alden and Sigrid receive an unexpected visitor. Gillian worries about her business.