Boardwalk Empire is a period drama focusing on Enoch "Nucky" Thompson (based on the historical Enoch L. Johnson), a political figure who rose to prominence and controlled Atlantic City, New Jersey, during the Prohibition period of the 1920s and 1930s. Nucky acts with historical characters in both his personal and political life, including mobsters, politicians, government agents, and the common folk who look up to him. The federal government also takes an interest in the bootlegging and other illegal activities in the area, sending agents to investigate possible mob connections but also looking at Nucky's lifestyle—expensive and lavish for a county political figure. The final season jumps ahead seven years, to 1931, as Prohibition nears its end.
Following his battle with Gyp Rosetti, Nucky makes a peace offering to Joe Masseria while working the odds with Arnold Rothstein. While Chalky is busy running the Onyx Club on the Boardwalk, the impulsive Dunn Purnsley clashes with a booking agent. Fresh-faced Federal Agent Warren Knox arrives in Atlantic City to learn the ropes from Agent Sawicki. Gillian seeks custody of her grandson, Tommy, while trying to find a “good” man to keep the Artemis Club afloat. Eli’s college-age son, Willie, turns to Nucky for career advice. Al Capone enlists his brothers, Frank and Ralph, to help him expand his business in the Chicago suburb of Cicero. Richard Harrow returns to his violent ways.
Now working as a “delivery man” for Dean O’Banion, Van Alden is ordered to keep tabs on Al Capone, joining the ascendant mobster to help keep Cicero voters “informed” at a political rally. Dr. Valentin Narcisse arrives in Atlantic City, clashing with Chalky and Nucky over the loss of an employee. In Washington, Agent Knox aligns himself with J. Edgar Hoover, acting director of the Bureau of Investigation. Harrow fails to finish a job in Milwaukee, while Eddie demands a promotion after 11 years of service. With a business opportunity on the horizon, Nucky departs for Florida.
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.
Nucky gets to know Arnold Rothstein over a card game, and sizes up an unexpected partner for his Tampa land deal. A newly promoted Eddie Kessler gets detoured after making a train-station delivery. When one of their collectors is incapacitated, Al and Frank Capone decide to finish his route, enlisting a reluctant Van Alden to tag along. Agent Knox fills Hoover in on the Thompson operation, vowing to exploit Nucky's weakest link. Chalky is intrigued by Daughter Maitland’s presence. Dunn Purnsley goes to Harlem, and is courted by Valentin Narcisse. Willie and his college roommate, Clayton, carry a practical joke too far.
Nucky heads to Philadelphia to deal with the fallout from Willie’s liquor fiasco. Eddie Kessler is detained by Agent Knox, who uses physical and psychological tactics to elicit information about Nucky’s organization. A desperate Gillian turns to an unexpected source to score a fix, and is rebuffed in her attempt to reunite with Tommy. In Cicero, Van Alden is recruited by the Capones to pay an election-day visit to local factory workers opposed to the Capones’ candidate of choice.
After a cautious reunion with Margaret in New York City, Nucky heads to Tampa to work out details of his land deal with Bill McCoy. While there, he reconnects with Sally Wheet. Richard Harrow returns to Atlantic City and makes a confession to an ailing Paul Sagorsky, who encourages him to get over the past and return to Julia and Tommy. Having lost J. Edgar Hoover’s confidence, Agent Knox finds a new way to gain Eli’s trust. Meyer Lansky decides to go it alone on the Tampa land deal when Lucky Luciano gets spooked by an investor, Vincenzo Petrucelli, who has ties to Joe Masseria in New York. Chalky takes his frustrations out on Daughter Maitland.
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.
Nucky’s shipment from Florida arrives with some unexpected cargo. Eli is backed into a corner after Agent Knox uncovers some new dirt on the Thompsons. After talking things over with Nucky, Willie reconsiders his recent life choices. Chalky throws down the gauntlet toward Valentin Narcisse, upholding his leadership on the Northside.
Nucky refuses to back Chalky in his conflict with Narcisse. Julia considers her options after Gillian makes her custody case in court. In Cicero, Van Alden stands up to Capone and comes clean to O’Banion. Rothstein looks to trade an insurance benefit for a cash infusion.
Tipped off by Sally Wheet, Nucky enlists Eli and Agent Knox to check out the extra cargo in Florida shipments. Margaret ponders a deal with Rothstein to improve her family’s living conditions. Capone grows suspicious of Torrio; Nucky faces a dilemma; Eli makes peace with Willie.
Nucky takes inventory after being warned of a skunk in his cellar. On the lam following a close call, Chalky and Daughter take refuge outside Wilmington with Oscar Boneau, his mentor from years ago. Gillian considers selling her mansion and starting a new life with Roy Phillips.
In Cuba, Nucky presses a U.S. senator on Prohibition's repeal and eyes a business deal with a rum magnate. Margaret witnesses a casualty of the Great Depression. Luciano makes a bold career move. Chalky catches a break to get out of a bad situation. In a flashback to 1884, a young Nucky makes an impression on the Commodore.
Nucky pays a visit to Johnny Torrio. Eli and Van Alden must recoup money lost during a federal raid.
Chalky heads in a new direction. Margaret finds herself in a quandary.
Nucky and Margaret find common ground in Atlantic City. Luciano encounters an old adversary.
Chalky returns to Atlantic City. Nucky arranges a meeting with Maranzano. Eli reunites with June.
Federal agents enlist the help of Eli and Van Alden to take down the Capone organization.
Nucky tries to hold onto his assets in Atlantic City. Willie and Eli become involved in Nucky's war.
Nucky makes a resolution; Margaret considers a hospital expansion; Gillian builds a new business; Van Alden stumbles onto a confrontation.
Nucky mixes business with pleasure in New York; Margaret's altruism is met with resistance; Gyp tries to establish a strategic foothold; Chalky and his daughter disagree about her future, Eli gets a new boss.
Nucky reluctantly joins Margaret to receive a high honor from the church. Gyp makes a one-time deal with Nucky. Van Alden regrets trying to socialize. Margaret pitches her women's clinic. Lucky and Meyer protect their investments.
Nucky and Owen hide out. Mickey makes a critical decision in Nucky's absence. Tensions rise in the feud between Capone and O'Banion.
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.
Nucky is detained in Washington. Margaret copes with Teddy's demons. Owen investigates a suspicious fire. Richard makes a connection with a veteran and his daughter.
Eli hopes to get back in Nucky's good graces. Gillian tries to improve her future. Gyp asks for a blessing in New York.
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 vows to eliminate his nemesis. A high-profile bootlegger is arrested. Emily's birthday party is scaled down. Gillian blames Harrow for Tommy's trouble. Gyp decides to buy a town's silence.
Nucky tries to keep Jess Smith quiet and neutralize Gyp's alliance with Joe Masseria. Margaret considers an alternate future. Van Alden's side business backfires. Gyp's shipment is short. Chalky sees an opportunity.
Gyp makes his move in Atlantic City, forcing Nucky and Eddie to seek refuge with Chalky. With the Artemis Club overrun by Rosetti thugs, Gillian contemplates a business future that includes an unwelcome partner. Luciano makes a deal he’ll come to regret. Harrow loses his house privileges.
A gang war erupts in Atlantic City, with Gyp and nine men of his own and 40 of Joe Masseria's against Nucky, Eli, Capone's Chicago crew, and Chalky's men. Gyp's holed up at Gillian's; Nucky at a lumber yard. It's a war of attrition, with Nucky having to keep Capone and Chalky's men from killing each other while he negotiates with Rothstein by phone. Rothstein runs his own game with Luciano, Lansky, and heroin, Margaret calls on an older couple, and Richard is out there somewhere focused on protecting Tommy.
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.
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, 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.
After his father Ethan is injured in a fall, Nucky uses the opportunity to refurbish his childhood home. During a visit to an army hospital, Jimmy befriends disfigured vet Richard Harrow, whose sharp eye may prove invaluable. Chalky considers a liquor proposition that would eliminate middle man Nucky. Van Alden gets a valuable lead on the woods massacre. Angela finds companionship in Jimmy's absence; Luciano and a friend cut a deal with the D'Alessio brothers. On a double-date with new friend Annabelle and her beaux, Margaret tries to get Nucky to open up about some painful childhood memories. On a tip from Capone, Jimmy eyes payback against one of Sheridan's thugs.
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 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 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 has emotionally charged conversations with both Margaret and Eli over his business and the killing of Margaret's husband, leading to dramatic shake-ups in his personal and political lives. Jimmy meets with his dying father for the first time in years, and in the process reevaluates his relationships with both his parents. Van Alden and Agent Sebso's working relationship is pushed to the limit by Van Alden's suspicions, ending after a definitive confrontation at a river baptismal. Angela prepares to flee from Jimmy to Paris, but learns her support system is not as strong as she believed.
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.
Despite being publicly embarrassed by legal trouble, Nucky's biggest concern may be holding on to his private army in a battle for control of Atlantic City. Meanwhile, Jimmy pitches a business deal to Arnold Rothstein in New York, but may have more in common with Meyer Lansky and Lucky Luciano. Margaret draws on past experience to retrieve some valuable information, Chalky clashes with a fellow inmate, and an Irish nationalist comes to town looking for support.
The Commodore throws his weight around with the Coast Guard to disrupt the delivery of alcohol, which creates an opportunity for Owen to demonstrate his value to Nucky and a struggling casino. Meanwhile, a letter dredges up painful memories for Margaret, Nucky calls in a favor with the U.S. attorney general, Meyer and Lucky learn the price of power, and Lucy searches for ways to cure her loneliness.
During Mayor Bader's birthday party, Nucky's attorney comes up with a solution that might help Nucky in his election-rigging case. With a liquor surplus likely, Jimmy and Mickey travel to Philadelphia to search for buyers while Nucky makes a surprising deal to try and get alcohol into Atlantic City. Eli is concerned over the Commodore's ability to lead. Margaret decides to give her staff bonuses. Chalky is cornered while at work and home. Owen takes his munitions expertise and uses it. Van Alden's agents set their eyes on Mickey's warehouse. Richard lets his guard down in the name of art. And Gillian thinks of the first time she was ever with the Commodore.
After a dispute with his enemies, Nucky brings in a surprise guest speaker at a Memorial Day dedication for war veterans. Running out of options, Eli questions his allegiances, but it ends up sending him deeper into the hole. Nucky hires a new lawyer. Jimmy is taught a lesson by one of the Commodore's men that he won't soon forget. Richard decides to go hunting in the forest.
Nucky tries to set up a risky liquor delivery to Philadelphia. Later, Attorney General Daugherty sees a problem that might complicate Nucky's case. Jimmy finds a new mentor in Leander Whitlock who is the Commodore's lawyer. Margaret finally goes to confessional. Lucy continues to struggle with loneliness while Van Alden tries to settle his conscience.
Margaret has a family reunion. Owen gets even with a rival. Van Alden recognizes the hierarchy of the law. Jimmy wonders how a decision will affect the future.
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.
Arriving in Belfast, Nucky and Owen meet with IRA leaders with a barter proposition. Meanwhile, in Atlantic City, Jimmy and his partners broker a deal with George Remus for his “medicinal” alcohol before taking a break to see/hear the Dempsey fight. Margaret frets when Emily’s illness takes a turn for the worse. Dunn Purnsley orchestrates a worker’s strike at the Ritz. Waxey Gordon takes action against Manny Horvitz in Philadelphia, and Randolph and Clifford Lathrop grill Halloran about Nucky’s past.
Undercut by Nucky, who has a new source for alcohol, Jimmy and his partners look to cultivate a new revenue stream in Atlantic City. With the peak tourist season approaching, the Commodore’s crew faces a choice in dealing with the city’s striking workers; either negotiate or fight back. Fearing that sin has led to misfortune, Margaret approaches Father Brennan with an “act of devotion.” Nucky gets a new lawyer. Van Alden faces an uncertain future. Manny Horvitz takes exception to a debt payment.
With Margaret pondering the consequences of sin, Nucky prepares her for a worst-case scenario as Esther Randolph plays out her trump cards. Jimmy revisits his college days with Angela and Gillian. Mickey Doyle chafes at sharing his liquor profits. Eli refuses to cop a plea. Nelson's past comes back to haunt him, tipping the scale in Nucky's favor.
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.