Broadchurch to The Politician's Husband: Tennant's extra... we just can't get enough

You do wonder how an actor must feel after a week such as David Tennant has had. No sooner had he cleared up the child murder in Broadchurch, than he was out of that shabby suit and off to the barber's for The Politician's Husband. From one unassailable hit to a classy fable of political hubris. From ITV to BBC. From ageing Bieber cut to silver fox.

Whatever satisfaction Tennant is taking, you know there has been a gaggle of ITV drama executives high-fiving their way through a dozen power lunches since it became clear the drama had so many viewers in its grip. And quite right too. The success of Broadchurch (ITV1, Sunday ***) is cause for rejoicing. Just shy of nine million viewers tuned in to its concluding episode on Monday. The last time ITV had the country in thrall to its quality drama output was the era of Cracker and Prime Suspect, two decades ago. (In my view, Downton Abbey hardly cuts it as "quality".)

The final episode took the Cluedo route – which means there's a spoiler coming up. As a flashback spelled out with a fastidiousness worthy of Crimewatch, it was Joe Miller (Matthew Gravelle), in the beach hut, with his own hands. And we came to know this information, after 59 days of the investigation and seven and a half hours of drama, because Joe couldn't stand the pressure of it all and just sort of gave himself up. This being prime-time fare, he gave himself up with some ruse involving a mobile phone signal that seemed to be reaching for dramatic suspense rather than psychological realism.

Still, we weren't to take this dénouement at face value, were we? Actually, we were, to clear the decks for the impact of Joe's confession on his community. Or to get ourselves comfy on the sofa for the reaction of Detective Sergeant Ellie Miller (Olivia Colman) to the news that most of Britain seemed to have cottoned on to a few weeks ago: that her husband is a child murderer.

She wretched, she stumbled, she gurned – all of DS Miller's chippy energy dissolved before us. The cynicism of her boss DI Alec Hardy (David Tennant), the mistrust that seemed to consume the town, it all burst from Colman's contorted features. It was like Picasso's Weeping Woman. Except in a police cell. ITV says Broadchurch will return. Let's hope that Colman is still on board.

But what am I thinking? This is David Tennant Week! In The Politician's Husband (BBC2, Thursday ****) he plays Aiden Hoynes, a frontbench thruster whose ambition succeeds only in his being outmanoeuvred by his old friend and political ally Bruce Babbish (Ed Stoppard) and ousted from the Cabinet. Then Hoynes's wife, Freya (Emily Watson), is elevated in his place. This three-parter is an update by Paula Milne of her own 1996 drama The Politician's Wife, and there's a veneer of up-to-the-minute references: advisers frantically monitor Twitter, Andrew Marr is mentioned, and Hoynes admonishes his wife's footwear-choice for being "a bit Theresa May". Does Milne have a real-life political couple in mind?

In fact, it's more of a fable – as I recall no political parties are named – and quite a self-important one. Did it need to open with a Latin homily? Corruptio optimi pessima is a bit of a give-away too: corruption of the best is surely the very worst of all.

What's already clear is that nothing good will come of Freya's ascent. We can sympathise that she has to suffer her husband's patronising assumption that her first duty in her new Cabinet post is to bring about his political comeback. But she's vulnerable to the lure of power too, and the way she was perving over the Cabinet table in No 10 at one point doesn't bode well.

Predictable, then. But there were pleasures along the way. I liked the wrinkle in the couple's apparently perfect family, that their son, Noah, exhibits obsessive compulsive tendencies – a potent reflection of the single-mindedness of a driven father. Tennant plays Hoynes well, with tight-jawed aggression, and Watson is typically excellent. In the face of his Machiavellian ranting, a double-blink tells you all you need to know about her frustration and brittle patience.

And those names are worth savouring: Aiden Hoynes and Bruce Babbish. Like many political careers, they start well and end up a bit weird.