Matthew Weiner’s “Mad Men,” about which there is always too much and not enough to say, returns for its sixth (and penultimate) season Sunday night on AMC with a two-hour episode. Right away, you can tell we’re in for a divinely morose and contemplative season — a real wallow. You can feel the show getting heavier, heavy even for “Mad Men.”

Whatever brightness and momentum the show caught last season during those pastel-hued rays of 1966 and early ’67 have been banished by a familiar gloom. We’re a long way now from “Zou Bisou Bisou” (even with Lane’s suicide, we’ll one day look back on Season 5 like it was an afternoon in the park), and it’s clear that when we watch the show, we’re meant to be thinking only of death from here on out. Death is more important than the tiny cracks in the old social order. Death is more important than the American cultural shift. Death is more important than furniture.

Where are we in the story? When?

The preview disc arrived with Weiner’s usual entreaty to critics to not tell anything. But early on, a character mentions that Alabama is going to play Texas A&M in the Cotton Bowl, so I’ll let you look it up while I reach over and press my oft-used SPOILER ALERT button. I’ve never understood why talking about the precise date and general outlines of “Mad Men” has to be such an issue when the real beauty of the show resides in faint subtext and customized interpretation. Part of understanding “Mad Men” is to realize that it is made by a control freak and consumed by a sophisticated audience that watches television like control freaks.

We find Don Draper and his wife, Megan (Jon Hamm and Jessica Paré), in pre-Christmas bliss on the beach at the Royal Hawaiian Hotel, which is Don’s new ad account. Their bliss is not uneasy or tense; they are free, by comparison, of the same ennui the writer Joan Didion once experienced at that same hotel beachfront, also in the late ’60s, when she wrote: “We are here on this island in the middle of the Pacific in lieu of getting a divorce.”

Don’s beach reading is Dante’s “The Inferno.” (Well of course it is, when really he ought to be reading Didion.) Sitting at the bar during one of his insomniac brooding sessions, he converses, at first unwillingly, with an Army private on R&R from Vietnam, and from that point forward, the entire cast might as well be wearing skeleton leotards and chanting booga-booga for two hours.

In this episode (titled “The Doorway”) we find these Silent Generation (and older) characters impulsively discarding otherwise meaningful objects — a Zippo lighter; a violin; a jar of water from the Jordan River — that might let death or disappointment in while their children and second spouses and younger colleagues blithely transition to the future.

Imagine a rerun of Norman Lear’s “All in the Family,” roughly contemporaneous in 1971, slowed down so much that one 30-minute episode took 10 hours to watch, almost to the point of immobility. Put Don Draper in Archie Bunker’s chair. Let the Meatheads and Glorias and Ediths and Jeffersons swirl around him, come and go, a constant and nagging reminder of how his world is no longer his. The gurgling toilet would elongate into an atonal funeral dirge. It’s the same show.

By which I mean the characters have entered a new circle of “Mad Men’s” inferno: Roger Sterling (John Slattery), now couch-prone in sessions of psychoanalysis — finally — is dealt some grief of his own; Betty (January Jones) ventures into the cold mayhem of the East Village on a hunt for a teen runaway, encountering a tenement full of hippies who regard her with utter contempt. (Someone please write Betty into the next staging of “Hair.”)

Elisabeth Moss’s Peggy Olson, meanwhile, seemingly thrives in her new job at a rival agency, trying to work last-minute miracles on a client’s Super Bowl ad crisis while, back at Sterling Cooper, Don seems to be losing his touch: Moved by a sort of transcendent experience at the Royal Hawaiian, he comes up with an ad pitch that is eerie and existential — and bluntly perfect.

“It’s a little morbid,” the client tells Don.

“Well, heaven is a little morbid,” Don says. “How do you get to heaven? Something terrible has to happen.”

“We sold death for 25 years with Lucky Strike,” Roger tells Don later. “You know how we did it? We ignored it.”

“Mad Men” is that rare thing that can be as infuriating as it is perfect. I’ve gone back and forth (and run hot and cold) on it as much as a critic can; I warmed to it last season but feel a familiar chill this time. “Mad Men’s” real genius as a TV show is that it encourages you to tease out its meanings and themes, all of them personal. As it happens, “Mad Men’s” time frame has arrived at the year of my birth, which, oddly, does not make it feel closer or more real. In fact, it makes the show feel more remote, imaginary and somehow sadder than ever.

Cheer up — at least it’s on.

Mad Men

(two hours) returns Sunday at 9 p.m. on AMC.