At 25 and a relative newcomer to New York, Megan hasn't yet figured out the infinite complications of Don's world, from his desire to keep work and home separate to the office politics into which she's been thrown. She doesn't realize that her impulsive planning makes everyone in the insecure firm assume by the late invite that they were inititally overlooked, and she has no idea that Don would be mortified by her sex-kitten rendition of "Zou Bisou Bisou" in front of everyone of importance from his professional life. (It resonates with an earlier party performance -- in French as well! -- when Joan has to tamp down her fury at her husband as she's corralled into accompanying herself on the accordian while singing "C'est Magnifique.")
Don's used to having a beautiful woman on his arm, but he doesn't want to be made to look ridiculous because of it -- doesn't want to see himself as Roger (John Slattery), who also married a young, fetching secretary, though the bloom seems to be off his relationship with Jane (Peyton List). ("Why don't you sing like that?" he murmurs to her. "Why don't you look like him?" she fires back.) There are echoes of Jane's drunken season-three plea to Don -- "You don't like me. I'm a nice person." -- in Megan's distress after the party, as she snaps at Peggy "What is wrong with you people? You're all so cynical, you smirk!" Roger's pursuit of Jane exasperated Don, but here he finds himself on the other side, insisting to his colleague, "We don't make fun of our wives here. Understood?"
Joan's story in the episode offered a different angle on being a woman in the workplace -- she's had her baby (Roger's, though her husband doesn't know) and is home caring for him with help from her undermining mother. (Joan, who never wastes a word, manages to conceil daggers in the suggestion that it might be time for her mom to head home: "I got my money's worth.")
She, too, has gotten a not-quite-perfect version of the life she always claimed to want, with a doctor spouse (away in Vietnam), a child, and a nice apartment painted to match her red hair. And yet all she can think about is what's going on in the cramped hallways of Sterling Cooper Draper Pryce, leading to the sweetest moment in the episode, as she stops by the office, certain she's going to hear she's been made unnecessary.
She and Lane haven't always gotten along, but the conversation between them in which he confesses to just how much a mess things have become without her was lovely and touching, not just because it featured the unflappable Joan letting her guard down in a rare moment of exhaustion and relief. "There would have been a cake, but you weren't here to arrange it," Lane soothes.
It's a pleasure to hear Joan admit that she actually loves and needs the office life she's always glided above and pretended she could easily live without, because we want her there, and because we love it, too. These people may totally be cynical, as Megan observed, and needy and competitive and deeply flawed. But aren't you so glad they're back?