Never thought we’d hear the phrase “mainframe footprint” on “Mad Men.” But the future is now.

SPOILER ALERT: Stop reading now if you haven’t season “Mad Men” episode 4, “The Monolith.”

At the end of this perplexing episode, all I could think of was how long Matthew Weiner and Co. have been waiting to use the Hollies’ jangly hit “On a Carousel” on the show. (It also reminded me that Graham Nash is a helluva singer.)

“Carousel,” of course, is a callback to the season-one finale that depicted Don in perhaps his finest form as an ad man, seducing client Kodak with a masterful pitch for a campaign for the slide-projector system. But even back then, Don the man is a big hot mess — apart from his family on Thanksgiving and being knee-deep in the subterfuge of Don Draper vs. Dick Whitman with everyone he knows.

After the ending of last week’s episode, when Don meekly accepts the punishing terms of his return to Sterling Cooper, I was surprised to see him fall apart so quickly again. But he has no allies in the office, other than Roger, who doesn’t have much of a base of his own and was preoccupied by his own family psycho-drama this week. Pete is pulling for Don from afar, as we see in this episode, but it’s not enough.

Granted, Don took a lot of body blows, from having to take orders from Peggy on the Burger Chef campaign to not getting the memo about the meeting to welcome company’s newest recruit — a computer system that, in Roger’s words, seems designed to make “Harry Crane feel important.”

The theme of this episode — “The Monolith,” written by Erin Levy and directed by Scott Hornbacher — seemed to be fear of progress, fear of the new. It also featured some interesting odd-couple reunions: Roger and his ex-wife Mona, Peggy and Don. The Sterling Cooper ranks, particularly the creative team that lives by their wits, not number-crunching, were unsettled by the arrival of the phone booth-sized computer (a nod to the monolith in “2001: A Space Odyssey”?).

To underscore the disruption, the need to find a space for the computer wiped out the “creative lounge” area where so much of the brainstorming was done. It also turned that area of offices into a construction zone, complete with the unnerving sounds of power saws and hammers and such. The agency is literally under construction again, and who knows how many of the existing people will make the cut? Maybe the computer will decide — at least that’s the fear evidenced by Ginsburg’s bad reaction to the computer and his desire to save the couch from the lounge area. At first Don tries to help him move it away, but he backs away at the first sign of resistance — in the body of Stan.

A parallel theme of eschewing technology and modernity unfolded with the plot thread involving Margaret, Roger and Mona’s daughter. She’s joined a commune upstate, where happy hippies in a beat-up Victorian house frown on electricity and try to move with the Earth’s cycles — and engage in plenty of sleeping bag hopping.

Usually, Roger would be down with that but not when it involves his little girl, the mother of his grandson, and a hairy guy who probably didn’t smell so great. The effort to retrieve Margaret (aka Marigold) brings Roger and Mona back together for a road trip. We get a reminder of why they were so miserable together, and how that extended to Margaret during her childhood. Margaret has no problem reminding both of her parents how unhappy their home was — bottle of gin for mom, parenting-by-secretary for dad. Her bitterness belies her claim from the first episode of this season that she had “forgiven” her father for his transgressions — a scene that was the first hint that Margaret was going off the rails.

Roger at first tries to woo her as a client, even going so far as to smoke a joint with them and stay the night. But in the cold light of the morning, Margaret has still abandoned her young son, Ellery, and there’s no amount of mumbo-jumbo philosophy that can make that OK. His humiliation as a father is complete when he and Margaret wind up rolling in the mud as she fights his effort to forcibly remove her from the compound. Watching Roger Sterling walk away from his daughter, muddied and bowed, was as painful as watching Don fall apart, again.

Don takes a similar woo-the-enemy approach with Lloyd the Lease-Tech guy. Don probably hates that computer more than anybody in the office, given his insecurity. But he turns on the Draper mystique when the man in charge of installing the future winds up in his office asking him for advice. Don’s instincts about pitching him for business are spot-on, but with Roger out of pocket he has to go to the oldest guy in the building, Bert Cooper, who can’t see it.

Like Joan, Bert is getting a perverse pleasure out of serving Don his comeuppance on a platter. Don responds with willful insubordination toward Peggy and seeking solace at the bottom of a bottle. That was quick. Thank goodness Freddy Rumsen is a generous soul.

Don’s level of discomfort is amplified now that he’s been pushed into the haunted office where Lane Pryce committed suicide. He finds the New York Mets pennant shoved under a couch — Pryce’s enthusiasm for all things New York even extended to the hapless ball club. Except that this is now 1969 — year of the Miracle Mets when the team defies all gravity to win the World Series. If Don finding that pennant isn’t a symbolic reference to things to come, I’ll eat my catcher’s mitt.

More observations:

** We need more Pete. He’s upset at learning in a roundabout way through a chance meeting in California that his father-in-law has had a heart attack. A sign that Trudy really has cut him off. I still don’t get Pete’s girlfriend.

** The sight of Harry Crane and Jim Cutler beaming smugly in hard hats makes me sure something bad is in store for both of them.

** How Cutler gained so much power is beyond me. He cannot prosper by backing Lou.

** Ted can stand up to Cutler, even from afar. Cutler wants him back in NYC probably because he senses that the Don-Roger dynamic could overpower him, eventually.

** Lloyd’s mini-speech to Don about the “God-like” possibilities of computers was heavy handed. Don saves the moment with his delivery of the line: “What man lying on his back counting stars ever thought of a number?”

** “Do the work, Don.” Words to live by from Freddy Rumsen.

** Love the relationship between Roger and his secretary Caroline.

** Who writes taglines without first settling on the strategy of what it is they need to impart? Lou must go!

** That was a weird aside from Pete regarding Peggy’s womanhood (“whatever she is”) during the conference-call discussion of putting her on the Burger Chef account.

** More Betty, more Megan, please. Both January Jones and Jessica Pare have been so good in their brief appearances to date.

** Just for fun: