Draper, Sterling Cooper's creative director, was jumping ship with a few other expats from that advertising agency to form their own shop camped in a Manhattan hotel suite. They and everybody else were still reeling from the assassination of President John Kennedy just a month earlier.
And "Mad Men" viewers were left to eagerly await the return of this zeitgeist-seizing drama, to find out how its bygone world would rise from the rubble.
Answer: Engrossingly, in unexpected, chancy ways, adhering to the AMC series' steadfast lack of formula.
You can check last season's finale, available online at the AMC website.
Then the fourth season begins Sunday at 10 p.m. EDT, setting the stage for a new round of discovery with the episode's first words: "Who is Don Draper?"
A mighty good question, but one aptly dodged by the magnetic, enigmatic and tormented Draper (series star Jon Hamm), who, it turns out, is being interviewed by a writer from Advertising Age magazine. Draper doesn't like this kind of grilling. He believes his work should speak for him.
Almost a year has passed. It's nearing Thanksgiving 1964, with the strain of the holidays soon to be visited on many of the characters.
By now, Don's frosty ex-wife Betty (January Jones) has remarried. Don has largely lost custody of their three kids and dwells in a somber flat in Greenwich Village where, at least on Thanksgiving, he prefers the company of a hooker to more traditional companionship.
The scrappy startup agency (whose team includes characters played by co-stars John Slattery, Elisabeth Moss, Vincent Kartheiser, Christina Hendricks, Rich Sommer, Jared Harris and Robert Morse) has lately taken space in Rockefeller Center.
And, as ever, Don is pitching new business with silver-tongued defiance, as with Jantzen swimsuits, which, he declares, must meet the bikini craze with a less prudish marketing strategy. Bluntly challenging the stubborn Jantzen bosses, he poses this mind-twister: "You want women who want bikinis to buy your two-piece or do you just want to make sure women who want a two-piece don't suddenly buy a bikini?" Very smooth, Don!
In sum, "Mad Men" is back.
"I wanted to hit the ground running," says Matthew Weiner, the series' creator and guiding force, in a recent interview.
Weiner readily acknowledges the risk of exploding both the family life and the gorgeously realized circa-1960s workplace of his complicated hero.
"It was terrifying when I ended last season," says Weiner, "and terrifying when I started this season. But I'm invigorated by risk. As Don says in the pilot, 'Fear stimulates my imagination.'
"These people are going to have a whole new set of problems, and it will keep me from repeating myself. Why tell the same story again, about how bad Don's marriage is? We told that story. I also felt that we'd covered every inch and corner'' of the stage set for Sterling Cooper's offices.
Never fear: Reflecting the obsessive research that underpins "Mad Men," its producers reached back across the half-century divide to locate the "right" real-life address (the Time & Life Building) for the new Sterling Cooper Draper Pryce agency.
"We know what the square-footage cost in that building was in 1964, and what the floor plan was," Weiner says. "We investigated what they could afford with the accounts they had."
In the season opener, Don, who typically is simmering beneath his charismatic, cool veneer, seems more snappish and tightly wound than ever. But Weiner sees potential growth in the character who, up to now, has prevailed by fashioning a false identity and zealously tending it.
"There's a sense of hope with Don on his own, and a sense of relief that he is able to be himself on some level," says Weiner. "He is still living a lie, but there's a lot less constraint on what he can tell the truth about than there used to be. He has lost his coordinates: He is single and divorced, he is not a daily father anymore, he is looking for structure of some kind. Maybe now he'll have to really look at himself. That was the story I wanted to tell this season."
"Mad Men" was the long-nurtured brainchild of Weiner, who, while trying to sell the show, wrote for "The Sopranos" in its powerful last seasons.
But as someone who struggled even getting his dream project on the air, Weiner remains hard-pressed to explain the stir it's created since debuting in summer 2007.
It's been showered with Golden Globe, Peabody and Emmy awards (plus 17 more Emmy nominations this year).
Meanwhile, the cultural link Weiner always sensed between the '60s and today has clearly been confirmed, infusing "Mad Men" a cocktail of hope, anxiety, delusion and disgruntlement with startling immediacy as well as retro chic. The shorthand terms "Mad Men" and "Draper-ish" are fraught with meaning, even among those who have never seen the show.
"I don't know what the cause of it is," muses Weiner, marveling at the "Mad Men" phenomenon. "For me, the show is very much a process of how I feel."
Sunday, the process resumes wondrously.