It's Memorial Day weekend, 1966, and Don's children are spending the holiday with him and his new wife, Megan, at the Drapers' new and stylish Manhattan apartment. Pete struggles at SCDP because of Roger continually attempting to undermine him through his accounts. Joan, on maternity leave after giving birth to her son, Kevin, has a helping house-guest in her mother. Megan throws a surprise birthday party for Don at their apartment with all of Sterling Cooper Draper Pryce in attendance, except for Joan.
When Betty sees a doctor to try to get a prescription for diet pills, he discovers a nodule on her thyroid. Although the nodule is later discovered to be benign, in the meantime Betty dwells on her mortality. Harry and Don go backstage at a Rolling Stones concert after a Heinz executive suggests trying to get the band to record a jingle for their ad campaign. Pete signs Mohawk Airlines as a client and gives the account to Roger. Roger asks Peggy to hire a new copywriter for Mohawk, so she interviews Michael Ginsberg. Don gives his OK.
Don runs into an ex-lover, and can't seem to escape her presence. Joan's husband, Greg, returns from his tour of duty in Vietnam only to reveal that he is being sent back for another year of service. Sally becomes frightened after reading stories on the Richard Speck murders, leading her step-grandmother to educate her on the concepts of fear and defense. Dawn spends the night at Peggy's apartment after becoming too afraid to return home because of racial violence near Harlem.
The episode occurs roughly two weeks after the previous one-around the time of Charles Whitman's shooting rampage at the University of Texas at Austin. Pete and Trudy host a dinner party in the suburbs, attended by Don, Megan, Ken, and his wife. Pete also attends driver's education classes where he views the film Signal 30 and flirts with a young woman. After watching England win the 1966 World Cup Final with expatriate Brits Lane attempts to land an account with Jaguar Cars, leading to a physical altercation with Pete.
Peggy becomes alienated after separate fights with her boyfriend Abe and a client. Roger and Jane take LSD with a group of intellectuals, altering how they see the world, and allowing them to speak honestly about their marriage. Don and Megan leave the office and take an impromptu road trip to Plattsburgh, New York, which doesn't turn out as planned after Megan takes offense at how Don treats her
Don is honored at a banquet for the American Cancer Society, but finds his professional reputation has been damaged. Megan comes up with a last-minute pitch to save the Heinz account. Peggy thinks Abe is going to propose to her, but he instead suggests they move in together, a decision that doesn't go over well with her mother. When Henry's mother hurts herself, Sally takes charge and then comes to stay with Don and Megan, who are already hosting the latter's parents.
Megan has second thoughts on her career path after the success with Heinz but finds it difficult to tell Don. Peggy, unwittingly caught between the two when Megan's lie comes to light, finally lets her frustrations be known to both Megan and Don. Pete finds that his increasing success at the firm does not fill the emptiness of his life. After a chance encounter one evening with a neighbor, he becomes obsessed with wanting to repeat the experience. Don returns to creative work full throttle, only to find the cultural changes of the 1960s have left him behind.
As a toxic cloud of smog hangs over New York City during Thanksgiving 1966, Betty's bitterness over her weight gain and Megan's youth and beauty leads her to tell Sally about Anna Draper in an attempt to cause turmoil between Sally, Don, and Megan. Don and Peggy are both irritated by Michael Ginsberg's rising star, and Don takes a devious approach to clip the wings of the young copywriter. Roger gets handed a new client (Manischewitz) by Bert Cooper, but Roger's decision that he needs more Jewish input to help him involves estranged wife Jane, with unfortunate results. Pete continues to resent that his brief affair with commuter-acquaintance Howard's wife, Beth, has ended.
Around Christmas, Lane is instructed by his London attorney to wire $8,000 to England within two days for back taxes, or else he risks being arrested. Lane, in fear, forges a check from Don. Harry meets with Paul Kinsey, who has joined the Hare Krishnas and has written a terrible spec script for the series Star Trek. Harry later pays for a trip for Paul to Los Angeles so Paul can try to break into scriptwriting. Joan is served divorce papers, and after an outburst to the office receptionist, she and Don go to a bar for a drink and have an intimate chat. Meanwhile, Pete reveals that Jaguar is again looking for an advertising firm, and that presentations will start in mid-January.
Pete asks Joan to sleep with a client to secure the Jaguar account. Meanwhile, the creatives work long nights to come up with the perfect pitch for the presentation. Don becomes furious after learning Megan could take a role in Boston. Feeling unappreciated, Peggy attends lunch with Freddy Rumsen, who encourages her to make a move. Later, Peggy meets with Ted Chaough, from rival firm CGC, and is offered chief copywriter position.
Don discovers that Lane stole money from the company and fires him. Sally has a rendezvous with former neighbor Glen Bishop, which ends abruptly. After Don expresses a yearning for more, Roger gets him a meeting with Dow Chemical, a client that could shape the future of the company. Lane falls into a melancholic depression and kills himself.
Don has a tooth ache that brings up painful memories of his brother as the firm looks to expand after a profitable quarter with Joan running the books. Roger seeks to expand the affair with Megan's mother and asks her to join him on a second LSD trip. Pete has a "last" encounter with Beth and finds she is receiving shock treatment. Megan struggles to find acting work and asks for Don's help getting an audition for a commercial at the agency to jump-start her career. Peggy is adjusting to her new agency and enjoying the challenge of an upcoming campaign bid for Virginia Slims, but still seeks Don's approval.