Series premiere. January, 1920. On the eve of Prohibition, Atlantic City's duplicitous Treasurer, Nucky Thompson, plots to make huge profits selling bootleg liquor. Later, he flirts with Margaret Schroeder, a pretty, pregnant wife with an abusive husband, and assures WWI vet Jimmy Darmody that his new job will lead to bigger things.
Investigating the heist now linked to Hans Schroeder, straight-and-narrow Agent Nelson van Alden pays a visit to Nucky; a concerned Nucky sends his brother, Sheriff Elias Thompson, to the hospital to sow the seeds of doubt in the widow Schroeder's mind, then lets Mickey Ward know he's being replaced by Chalky White in the bootleg-production business.
An unlikely witness threatens to expose the perpetrators of the woods massacre. As Nucky re-evaluates his relationship with Lucy, Margaret starts a new job. 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. Margaret impresses at Nucky's birthday party, while Lucy tries to protect her territory.
Nucky's attempts to usher in a joyous St. Patrick's Day are undermined by Eli, Margaret & Van Alden. 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. 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 Charlie Sheridan and the neighborhood of Greektown.
Nucky purges some bad childhood memories; Jimmy forges a new alliance with a fellow veteran. Chalky considers a liquor proposition which 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.
In the Season One finale, Nucky and Atlantic City brace for change on Election Day; Torrio brokers a deal between two nemeses, with far-reaching consequences; Jimmy ponders his future, as do Margaret, Agent Van Alden and Eli.