Set to Diana Ross’s Love Hangover, the preview of the final season, which starts on 5 April, is stylish, ironic and mysterious

As a rule, Mad Men previews do not spoil twists, reveal endings or offer much of anything in the way of character development. They suggest that Mad Men is less a TV programme than a weekly, period-piece fashion show starring a lot of genial actors whose director keeps telling them: “But now with more ennui”.

As meandering and erratic as the show can be, though, it’s also one of the most interesting shows of the ballyhooed and largely great golden age of TV. So while its previews defy the analysis that others encourage, they at least offer a welcome sight of old friends on the verge of the 1970s.

Life looks sweet for Peggy Olsen, sitting pretty in red and striding happily away from Pete Campbell, balding and bitter as he munches hors d’oeuvres. Good old Betty smokes and lounges, as blank an expression as ever, as Sally Draper does her best thoughtful gaze into the middle-distance.

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Don is sceptical and maybe a little annoyed – also par for the course – and Megan dons a pink, bell-bottomed ensemble disinterred from the attic of a proto-discophile. Roger Sterling is cultivating a trendy haircut of that resembles nothing so much as a toupee. He smiles smugly about it. The ad guys, decked out in plaid suits, laugh fraternally around somebody’s car, perhaps suggesting that the moody show will at last embrace its surreal sense of humour. Or more likely suggesting nothing at all.

And then there’s the other Mad Men standby: the contrapuntal, semi-ironic song, in this case Diana Ross’ Love Hangover, from 1976. What to make of it? A hint that the story will jump seven years ahead? A theme ripped from Thomas Pynchon about American disillusionment and nostalgia? A sign of disco balls to come?

There is only one thing to be trusted from this 30-second song and clip: the 70s are fast approaching and they bring bass.