Mad Men (Sky Atlantic) used to just be a hazy drama I'd eat my dinner to.

There'd be no question of eating whilst the horrific Walking Dead was on, as whatever lay on your plate would begin to resemble broken jaw bones and wet red intestines. Impossible, also, to eat along to House of Cards because all those devilish intrigues needed close attention. There'd be no chance of chowing down whilst watching Arrested Development (too high a risk of laughing and spluttering your chips onto the carpet) and certainly not with Breaking Bad as it required an awed silence, unaccompanied by chomping molars.

So what American series could I watch whilst having dinner? Mad Men was the only option as it was rather undemanding. There was no twisting plot to follow so you'd miss nothing if you had to duck back into the kitchen for the pepper.

But slowly, over the weeks and over the pasta, I became addicted to the show. I didn't even realise it was happening, because nothing was happening. Characters flirted and married. Some divorced and some drank and some had affairs and spectacular clothes, but there were never gasping, cliff hanger moments. You could always leave to get the pepper! It was when Mad Men was over, and the TV off, that I'd start to feel the lure of it. I wanted to slip back into the intricate world it had created. I wanted to be back in the office, the 1960s office of cigarettes and clattering typewriters.

The final series resumed tonight after its mid-season break and I'm already feeling a quiet sadness at the thought of it ending. The sense of impending gloom was reflected back at me from the episode itself, which was a dark and unsettling one. Mad Men may have been known for portraying the bright, swinging world of the 60s, but the show has now entered the 70s and everything is harsher. The sweet pastel shades and flouncing dresses have vanished, and the characters are now clad in greens, mustards, browns and greens, paired with loud patterns and big, airy hairdos. Already we're feeling the loss of innocence as America's new, discontented era begins.

The sense of unease, and of shifting space and time, was clear from the beginning. The episode opened with a dream sequence: Don is watching as a queue of young women sashay into his office to model mink coats. One of the women is his old flame, Rachel Katz. A few days later he's told Rachel has died of leukaemia, so was his dream of her simply coincidence or a more frightening omen?

Things get worse for Don. No longer as sharp and smooth as he used to be, he finds himself confused in a grubby backstreet café. He's convinced he knows the waitress, that he's seen her somewhere before. He returns to the café again, seeking an assurance or a memory from her. Whilst she's may step into the back alley with him, she can offer no advice except to suggest his mind may be upset by his bereavement. The episode ends with Don sitting alone at the café counter, pondering this. Again came the feeling that this was some kind of omen. Maybe a presentiment of a deeper tragedy to come?

So Mad Men isn't back with its old saucy, sexy, 60s colour, but has ripened into something darker and more troubling.