Midway through the "Mad Men" sixth season premiere (Sunday at 9 p.m. on AMC), Don Draper and Pete Campbell have a conversation that is not only like every conversation they've ever had, but about every conversation they've ever had, and how they are all the same. The show has been on for so long that everyone — Don, Pete, "Mad Men" creator Matt Weiner, the viewers at home — knows exactly how it works.

On many an aging series, that level of familiarity might become a problem. Even a classic series like Weiner's previous show, "The Sopranos," struggled at time with formula in its later seasons, to the point where the actors (who were often just as in the dark as their audience) would start guessing which one of their new co-stars would turn out to be, as James Gandolfini once put it, "the new Richie Aprile: the guy we yell at for nine months."

But the familiarity of "Mad Men" at this point only enhances the experience, because it's always been a show driven more by character and theme than by plot. What's going to happen next is never remotely as important as how and why it happens, which is why I've always felt that Weiner's handwringing over spoilers does a disservice to his own creation. There are at times surprises — Don's true identity, Peggy's baby — but the most interesting aspects of them are rarely the surprises themselves, but what they tell us about these people. Lane Pryce's suicide last season, for instance, would have been considered a shock before the season began, but by the time we got there, we'd witnessed the disintegration of this lonely man's life to the point where it would have been surprising if he hadn't hanged himself in the office.

Instead, "Mad Men" focuses on change, sometimes big (the rise of the counterculture, or how far Peggy's career has come), sometimes smaller (Harry Crane turning out to be a lazy pig), and sometimes unsuccessful (Don's attempt last year to throw himself into a happy, monogamous marriage to Megan). Don and Pete may say roughly the same words to each other over and over again, but they're not the same men each time, which makes the same conversation feel very different.

The premiere, called "The Doorway" is another two-hour affair checking in on the emotional state of Don Draper, but it doesn't feel in any way like a rehash of last season's "A Little Kiss." And as the series gets closer to its ending  (barring something unexpected, next season will be the last), it's become ever more aware of how much the times, and the characters, have changed — and how hard it is in those who don't want to change.

Without giving away the exact date (last season ended in April of 1967), we're now deep enough into the '60s that when Don has a meeting with the twentysomething writers and artists on his team, they appear to be from different species. They work in the same office, doing the same work, but they have nothing else in common. It’s not just the clothes and hair that have transformed, but entire states of mind.

(“Mad Men” has always distinguished itself from most ‘60s period pieces by taking the side of the straights. When we see beatniks or hippies, it’s almost always from the perspective of an establishment character like Don or Peggy or Betty.)

When we see Peggy, it’s clear she’s absorbed an awful lot from Don; other than context and Elisabeth Moss’s usual great performance, there’s virtually no resemblance between this Peggy and Don’s mousey secretary from season 1.

Again and again throughout the premiere, we see characters who have adapted to the changing times, and their own changed circumstances, and others who are trying to carry on as they always have, even as that position becomes less and less tenable with each passing year.

One of the show’s most entrenched — and funniest, thanks to the way John Slattery delivers every brilliant one-liner he’s given — characters is Roger Sterling, who at one point in the premiere tries to suggest that change is pointless. He compares the journey of life to going through doorways that are supposed to take you to new places, but inevitably leave you exactly where you were before.

Given what we’ve seen of Roger over the previous five seasons, it’s not surprising he would feel this way. But the world around him is changing, as are many of the people. The show that portrays them, on the other hand, both has and hasn’t changed. It’s arguably a richer, deeper, more experimental work than it was at the start, and it’s been willing to transform its characters with the time. (And the performances by people like Moss, Slattery, Jon Hamm, Vincent Kartheiser and others have only gotten better the more we and they have grown to understand these people.)

But it is, fundamentally, still “Mad Men” — which means it continues to be one of the most satisfying dramas in the history of the medium. It’s great to have it back.

Alan Sepinwall may be reached at sepinwall@hitfix.com