Don Draper has married his secretary Megan Calvet, who throws a surprise birthday party for Don and their co-workers. Megan has been promoted to copywriter, but struggles with Don's growing detachment with work, as well as her own unfulfilled dream of being an actress. Don's detachment also alienates Peggy, who is being made to train Megan, and Bert, who feels that Don has gone "on love leave" and does not care about his job or turning in quality work. Feeling insecure about her career ambitions, Megan ultimately quits her job at SCDP to pursue acting.
Don's slacking at work coincides with the arrival of a new hire, in the form of young advertising phenom Michael Ginsberg, who proves to be a rival for Don and Peggy, the latter of which is frustrated over Ginsberg being able to rise faster within the company. When the creative team are made to pitch advertisements for a snow cone company, Don purposely leaves behind Ginsberg's material in order to pitch his own campaign, which is ultimately chosen. Meanwhile, Roger struggles to remain relevant in the company as Pete schemes to steal his plush office for himself, and he begins to secretly pay Peggy and Ginsberg to produce material for him to pitch to clients. Roger also experiments with LSD, which has a profound impact on him and his own marriage to Jane; under the influence of the drug, the two confess that their marriage has failed. After Roger and Jane get a divorce, Roger begins an affair with Megan's mother Marie.
Joan struggles with single motherhood while Greg is overseas. When Joan discovers that Greg has signed up for another tour of duty in the Army Medical Corps without consulting her, she kicks him out of the house. Greg later files for divorce, and Joan fears that he will paint her as the villain in their divorce case. Further complications arise for Joan in the firm's pursuit of Jaguar as a client, when Pete discovers that the agency will get the account if Joan sleeps with the head of the Jaguar dealers' association. Pete arranges a vote behind Don's back, and Lane convinces Joan to take an ownership percentage of the company in exchange for sleeping with the client. The firm ultimately wins the account, but alienates Joan and Don from the rest of the partners and from each other.
Pete, having moved to the suburbs, begins to become more detached from his life and starts missing the big city. His relationship with Lane collapses and the two fight, with Lane beating Pete up in front of the other partners. Pete also begins a relationship with Beth, the wife of a fellow train commuter, who breaks off their brief affair out of guilt. Beth later tells Pete that her husband is forcing her to undergo electroshock therapy for her manic depression. Pete visits Beth in the hospital, but she does not remember him; Pete then confronts Beth's husband on the train, culminating in a fist fight. Returning home defeated and alone, Trudy agrees to allow Pete to rent an apartment in the city for overnight stays. Peggy, meanwhile, feels unappreciated for her work and decides to leave the agency for another firm in order to fulfill her full potential. Don attempts to keep her by offering her a raise, but ultimately concedes that Peggy has to leave him to continue out of his shadow.