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TWO years ago, in the first series, Dr Gemma Foster seemed to have it all. She was a fortysomething professional with a loving marriage, young son and a smart home in the fictional town of Parminster (some of it was filmed in the affluent Hertfordshire town of Hitchin).

When she discovers a blonde hair on husband Simon’s (Bertie Carvel) lapel at the beginning of the first series, her cosy world falls apart. Simon has been having a two-year affair with beautiful Kate Parks (Jodie Comer), the 23year-old daughter of their friends. Everyone seems to know except Gemma. Infuriating!

She does not take this well. Strategies involve sleeping with Simon’s accountant to elicit information on his finances and breaking medical ethics to entice patients to spy for her.

At an excruciating dinner party at the Parkses’ home during the series finale — which drew ten million viewers — she reveals Simon’s infidelity over pasta and salad, even informing Kate’s parents their darling daughter is pregnant

‘I’m a wolf tonight,’ she said, relishing the mayhem. Later, she pretends their son Tom has been kidnapped or killed, just to give Simon a fright. He knocks her unconscious for her troubles — securing him a handy criminal conviction and restraining order.

In the final scenes, Slimy Simon and his pregnant mistress moved to London, leaving Gemma to pick up the pieces of her life. And initially, she seemed to be doing just fine.

In series two, Simon, Kate and baby daughter Amelie return to Parminster. They are now married and Simon appears gloriously rich and successful; sharp-suited, smug and with a fabulous new house and swimming pool.

All he wants now, he says, is his son back and ex-wife Gemma out of the picture so he can reconnect with his old life. Not so fast, big guy. Dr Foster has other ideas.

Like many divorcing couples, they want to punish each other. Badly.

Meanwhile, the way they use poor Tom is shameful, though neither can see it through their haze of righteousness.

Writer Mike Bartlett has done a terrific job exploring the blasted heath of the post-divorce landscape; a place where friends must choose where their loyalties lie and casual affronts are hammer-blows.

Yet at the centre of this drama is the quiet, sad, dichotomy between the one who leaves and the one who is left behind.