It was only when I stopped drinking for two days a week as part of my commitment to the 5 and 2 fasting diet that I began to appreciate the complex relationship between late-night television drama and inebriation.

It was a July evening this year when I had my first insight into the matter. As it had been a fast day I’d resisted all alcohol and confined my eating to nothing more than a bowl of acidic blueberries and a 360-calorie microwave pack of pinkish nothingness masquerading as chicken piri piri. At around ten o’clock, as was customary on normal drinking evenings, I scanned the channels in search of one of those murder mysteries that are punctuated by commercials for river cruises, stair lifts and donkey sanctuaries.

I’m pretty eclectic when it comes to such programmes. As long as I have a very large whisky to hand then I’m as happy with Midsomer Murders as I am with Miss Marple or Inspector George Gently. Part of my pleasure lies in the scenery: the old country pub and the ancient vicarage and the neatly mowed village green and the police station with the bumbling sergeant behind the desk and, of course, the shadowy graveyard at midnight.

But what always has me reaching happily for the whisky bottle is the dialogue. I can barely wait for the Inspector to ask someone to “account for their movements” on the night that the retired army major’s head was found in a hatbox behind the post office. And I’m frankly on tenterhooks until the moment when the suspect axe-murderer is told that he will need “to come down to the station”. But what I’m really waiting for as I jiggle the ice in my glass is the magical moment when a possible suspect opens the honeysuckle-encircled front door of his cottage to find himself facing the avuncular inspector and his know-nothing sergeant who proceed to explain the reason for their visit. The camera slowly closes in on the face of the cottager. Only then does he utter the magical words: “You’d better come in.”

I’ve always regarded this ensemble of delights as quite enough for one evening. But on the day in question, the day of no alcohol and acidic blueberries and calorie-diminished piri piri, I slowly began to discover a wholly new dimension to my viewing. I noticed that the axe-wielding psychopath had a cigarette burn in his dining-room tablecloth, even though we’d learned earlier that he was not a smoker. But there was more. During the funeral scene when the vicar was busy burying the elderly widow whose late-night cocoa had been enlivened with a splash of strychnine, I noticed that, although it was a sunny day, there were spatters of mud on the boots worn by the exotic African stranger.

I mentioned these matters to my partner. “Many congratulations,” she said in the heavily ironic tone she tends to reserve for fasting days. “Now that you’re completely sober after nine o’clock in the evening for the first time in your life you’re actually following the story.”

“How d’you mean?” I asked.

“Well, instead of simply looking at the people and the places and listening for your favourite catchphrases, you’re trying to solve the mystery.”

“What mystery?” I said.

I decided to mention the matter to an assistant producer at the BBC who doubles up as an ego-centred psychotherapist. As I walked her back to her flat in Fitzrovia I asked casually if she found anything at all disturbing in the fact that I’d watched these late-night programmes for years and never until now wondered what it was that united all the places and people that passed before my eyes?

She was immediately reassuring. An inability to see such causal connections was, she told me, pausing on her doorstep, not at all uncommon among some of her elderly patients. They often had very clear ideas about who they were and where they were. And they also knew much the same about their relations and friends. But they could never quite work out why any of this mattered. They lacked all interest in other people’s purposes and predilections. They simply watched contentedly as the world’s scenery passed by.

Was there a special term for this condition?

“We call it ‘Losing the plot’.”

Was there any cure?

She gave me a solicitous look.

“I think you’d better come in.”