In the 1960s New York, Don Draper is a top exec at Sterling Cooper ad agency who’s working hard to keep the Lucky Strike account, whereupon he hires a new secretary, Peggy Olson, who is intimidated by the office manager, Joan Holloway, while Pete Campbell, an engaged accounts manager, takes an interest in her.
As Roger Sterling encourages Don to confide in him about his personal life, Don remains secretive about it. Pete goes on his honeymoon, while Peggy thinks about him. Betty decides to see a psychiatrist.
When Don meets Rachel Menken, a client, they become drawn to one another. As Pete comes back after his honeymoon, he broods over his past affair with Peggy.
Don feels undermined as Pete pitched his own ideas to a client behind his back, while Trudy, Pete’s wife, pushes him to buy an apartment in Manhattan. Betty makes the acquaintance of her new neighbor, Helen, and her little boy, Glen.
As Adam, a young man who claims to be Don’s brother and calls him Dick, approaches him, secrets are revealed about his past, whereupon Don tries to send him away. When Ken’s story gets published, Pete becomes consumed with jealousy.
As Roger gets tired of having to meet Joan in hotels, he suggests she gets her own place. Peggy is asked to write copy after she came up with a brilliant idea during a meeting. Don contacts Rachel Menken to pick her brain about an ad campaign to promote tourism.
When Don invites Roger to his home, a drunk Roger hits on Betty, which causes friction between the three. Pete gets into trouble as he trades a wedding gift for a rifle. Richard Nixon’s campaign staff arrives at the agency for a meeting with the execs.
Peggy and Pete’s relationship develops, while her work on the lipstick account turns out well. Don visits Midge and spends time with her and her beatnik friends, whereupon he recalls a distant memory about a hobo. As a new receptionist is hired, she takes a liking to Sal, but she doesn’t seem to be his type.
The head of another agency approaches Don, offering Betty a job as a model to ensure Don’s acceptance. As people in the office start noticing Peggy’s weight gain, Ken makes a nasty remark about her, which angers Pete, who comes up with an idea to help Nixon’s campaign against Kennedy.
Over Labor Day weekend a lot happens; Betty frets about the visit of her father and his new girlfriend; Roger invites twin sisters to spend the night with him and Don, whereupon he suffers a heart attack; Joan has a strange encounter with a friend; Don goes to see Rachel.
Peggy discovers another use for an ineffective weight loss device. As Roger suffers another heart attack at a business meeting, Don is made partner, which makes Pete jealous, whereupon the latter receives a package that was meant for Don by mistake.
On election night, the employees at Sterling Cooper throw a party while waiting for the results. Pete tries to blackmail Don, using the information he learned about his past and his true identity, to make him the new head of accounts.
Don quarrels with Betty over Thanksgiving, whereupon he learns what happened to his brother, Adam. Pete manages to land a big account for the agency. Peggy receives a promotion, after which she feels sick and goes to the hospital, where she learns something unexpected. Don pitches a new idea to Kodak.