I’M not saying my husband is in any way slow or dopey, really I’m not. The other day, in fact, as he hurled himself towards the radio, I was reminded of Sam Johnson’s magnificent Six Nations try against England when, dodging all obstacles, he hared over the line. In our house, rather than a herd of beefy defenders the opposition comprised a chair, table and toaster, and still he got there in time.

The reason for his sudden athleticism? A Radio 5Live commentator was about to give away the plot from the previous evening’s episode of Line of Duty. This pundit hesitated, and then decided it wouldn’t matter to reveal what had happened, because everyone had already seen it. “No we haven’t!” we yelled, and thankfully silenced him before he could draw another breath. In the age of iPlayer, how could anyone be so stupid? Having said that, he was to some extent right. I can’t remember when there was last a series that had the nation so in thrall that on a Sunday evening, lawnmowers, microwaves and mobiles fall quiet a minute before nine, as everyone settles down to watch.

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It tells you the hold this TV drama has on us that even a hint of what is to come is a spoiler. Quizzed on the Today programme about what we can expect in the final episode, Vicky McClure, who plays Detective Inspector Kate Fleming, refused to be drawn on anything except that they all hope there will be another series.

A police drama about an anti-corruption squad, now in its fifth series, it is the most watched programme of the year. The first episode attracted 7.8 million viewers, which is a tribute to the quality of the script and the actors. Obviously the script comes first, but Jed Mercurio’s superbly crisp, understated, powerful dramatisation would lose its punch in the hands of lesser actors than Adrian Dunbar, Martin Compston, or McLure. They – and others – are so convincing, it’s nigh on impossible to imagine they might be different in real life. Yet as Dunbar recently revealed, some of the Northern Irish expressions he suggested Mercurio introduce to his lines are an echo not of him but of his late father. “I’m calm. I’m bloody calm!” is not something you’re likely to hear him say off-set.

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Like all good writers, Mercurio does not spells things out or labour a point. He offers the bare minimum of what is needed to understand a scene. Much of the drama, indeed, lies in individuals’ expressions. When they do speak, though, it can be electric. Quite how interrogations around an office table can feel as tense as a courtroom grilling is hard to fathom, yet it is in the banal surroundings of police headquarters that Line of Duty really takes flight. The ambushes and murders around which the plot revolves are secondary to the intensity of these in-house inquisitions. When each series ends, for me at least, it is those encounters, with suspects, accusers and a folder of evidence, that have burned themselves into the imagination. Not since Tinker, Tailor, Soldier, Spy has the workplace seemed so sinister.

For too long, it’s been widely agreed that when it comes to TV excellence, America is way out in front. The Sopranos, The Wire, Breaking Bad, Homeland and Fargo all hammered nails into the coffin of British self-confidence when it comes to small-screen drama. It has become something of a cliché that we cannot hope to compete.

Line of Duty is not, of course, the only title to challenge that lazy stereotype, but it is among the most consistently fine home-grown programmes we have produced in recent years. How often do long-running series lose their lustre and their point as the episodes tick by? As scriptwriters strain to find fresh inspiration, the storyline grows increasingly weak, like a tea-bag passed from cup to cup.

Yet if anything, Line of Duty is getting better. A large part of that can surely be attributed to its integrity. The best writing, be it drama or novels, should never be cynical. There can be no pandering to an audience, but simply a commitment to tell a story that the author believes is important. As a result, the core of this series is a moral seriousness that raises questions crucial for our times. Perhaps it feels particularly compelling at this point, when British values are in exceptional flux.

An inquiry into those who maintain law and order who are neither above the law, nor always on the right side of it, Line of Duty is unflinching. When characters make tough decisions, I imagine many viewers wonder if, in the same circumstances, they would show similar courage or dedication. Well-versed in the weasely, venal ways of corruptible officers, we are also now alert for hidden motivations and meanings. If nothing else, this piece of fiction is a masterclass in taking nothing at face value. Like the musketeers of old, its cry could be “On Guard!”. You might take this not as a warning, but as a timely reminder.