Peter finds out that he will become a father, and has to break some bad business news to his father-in-law. Meanwhile an office focus group study for a client has some unintended personal consequences for Don.

Don has a wild lost weekend after winning a CLIO award, while Peggy is forced to work all weekend in a hotel room with an "uninspired" co-worker. Meanwhile, Peter is livid after he learns that the other partners hired Ken Cosgrove.

Megan throws Don a surprise birthday party, the war between Pete Campbell and Roger Sterling intensifies, and new mother Joan begins to wonder if she still has a job when she's ready to come back.

Megan is hurt by Don's negative response to the surprise party, Lane finds a missing wallet in a cab, Joan makes an unexpected visit to the office, and Pete makes a play for a bigger office.

Betty discovers that she has a tumor, Peggy interviews an obnoxious young copywriter for a job, the firm tries to get the Mohawk Airlines account back, and Don and Harry try to court The Rolling Stones to record a commercial.

As the partners consider expanding the office space, Don begins seeing ghosts and gets a request to advance Megan's acting career. Meanwhile, Peter's affair comes to a disappointing end, and Megan's mom briefly reunites with Roger.

Things quickly become awkward after the two firms merge, Don asserts his dominance over Sylvia, Pete has to deal with his ailing mother, and Bob Benson helps out a sick Joan.

Don is preoccupied with keeping Sylvia's son from being drafted, Peggy has a rat in her apartment, Don and Ted bury the hatchet, Pete has a problem with his mother's new nurse, and Sally makes an accidental discovery about her father.

Don renews his battle with Ted after seeing his relationship with Peggy, Sally interviews at a boarding school, and Pete discovers Bob Benson's background.

With his personal and professional life crumbling around him, Don positions himself for a change of scenery. Meanwhile, Pete's life is falling apart as well, thanks to the meddling of Bob Benson.

Don is on the outside looking in on the forced indefinite paid leave from his job. While Ted heads back to New York for a few days, Don takes the opportunity to head to Los Angeles in part to meet with a California transformed Pete while he reunites with Megan, who is living full time there while pursuing what looks to be a promising acting career. Back in New York, Peggy avoids Ted at all cost while she learns to deal with the new "Don", Lou Avery, most specifically on the Accutron account for which she is getting consulting help from Freddie Rumsen. On top of that, she is also having to deal with being a landlady to who she believes are overly demanding tenants. Ken feeling overworked leads to Joan taking over his scheduled business meeting with Wayne Barnes, the new Head of Marketing for Butler Footwear. Barnes' news results in Joan taking what she hopes will be seen as decisive and preemptive action in overcoming a potential problem, action which she hopes demonstrates that she deserves to be more than just a glorified secretary. And Roger meets with Margaret on her request, the two who have been somewhat estranged over the last few months. Their individual new approaches to life may result in a friction-free reunion.

Peggy has second thoughts about her Burger Chef campaign when Don is asked to pitch her commercial, Pete visits his wife and daughter on his trip back to New York, and Bob Benson gets some surprising news from G.M. about the account.