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.

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.

Nucky is shocked by an insurrection among his inner circle. An attack threatens Chalky's life and livelihood. Margaret deals with her son's problems. Angela vies for Jimmy's affection. Van Alden shows his wife around town.

In the wake of a personal loss, Nucky solicits advice on his political situation from Arnold Rothstein, while Eli remains suspicious of his brother’s actions. Nucky finds himself in the unfamiliar position of showing gratitude toward a federal agent. Angela meets a new friend at the beach, and eavesdrops on her husband’s treachery. Nelson gets a nanny. Nucky approaches Owen with an overseas proposition. Jimmy flexes his muscles, at Mickey Doyle’s expense.

Esther Randolph's case is weakened by the sudden death of one witness and the sudden marriage of another. Jimmy turns away from the influence of others, including his mother. Nucky, acting the family man to Margaret, faces a difficult, bottom-line business decision.

Nucky makes a resolution; Margaret considers a hospital expansion; Gillian builds a new business; Van Alden stumbles onto a confrontation.

Gillian makes her peace with Jimmy's death; Van Alden attends a meeting with Capone and Torrio; Nucky makes an offer to Andrew Mellon; Margaret makes a stand on birth control; Nucky and Owen shop for a pony; Billie considers a future in film.

Nucky makes a deal with Rothstein as his war with Gyp Rosetti reaches its climax. Season finale.

Nucky has reservations about investing in a Tampa land deal engineered by transplanted bootlegger Bill McCoy. While there, he meets speakeasy owner Sally Wheet. Valentin Narcisse books a sexy jazz singer, Daughter Maitland, to perform at the Onyx, and gauges Dunn Purnsley’s allegiance to Chalky. Willie, Eli’s son, turns to Mickey Doyle and his bootleg warehouse to elevate his social standing at college. Gillian agrees to pose as Roy Phillips’ wife to impress a possible partner. Harrow tries to bury his past, though not before taking care of some unfinished business with Emma.

Eli confronts Nucky after Willie makes an unpopular decision. Margaret has a surprise encounter at her job on Wall Street. Joe Masseria orders Lucky Luciano to return to Tampa and deal with Vincenzo Petrucelli. Gillian goes cold turkey, with Roy Phillips’ help. Still seething over his brother’s death, Al Capone tries to get Johnny Torrio to turn on rival gangster Dean O’Banion. As Chalky White and Daughter Maitland’s relationship deepens, Valentin Narcisse exerts his control over Dunn Purnsley and the Northside.

Eli braces for the worst as Agent Knox launches his plan to bring Nucky down. Capone and Torrio reach an agreement in Cicero. Chalky looks to settle scores. Richard steps forward to save Tommy, at Gillian’s expense

Nucky pays a visit to Johnny Torrio; Eli and Van Alden must recoup money lost during a federal raid by Eliot Ness; Willie has a job interview; in 1884, young Nucky deals with the death of his sister.

Nucky plans to relocate to Manhattan; Margaret impresses Nucky and a grain company stockholder.