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Tensions hit a peak on the thin blue line

IS she guilty or innocent? This is the question gripping those of us who've been following the brilliant second series of BBC2's Line of Duty for the past five weeks. All will be revealed in tomorrow night's final episode.

Don't be surprised if Line of Duty's creator and writer Jed Mercurio, who's been delighting viewers with unexpected twists, switchbacks and cliffhangers, has a few last-minute tricks up his sleeve.

You shouldn't, by the way, read that as some kind of tease or clue. I'm as much in the dark as you. In an unusual move, the BBC decided not to provide TV critics with a preview of the final episode.

When the broadcaster is afraid that even trusted reviewers employed by reputable newspapers might leak spoilers, it reflects just how big a deal Line of Duty has become.

The audience has been increasing week on week and the superlatives growing ever more lavish.

DI Lindsay Denton, played in a truly revelatory performance by Keeley Hawes, may well turn out to be a devious mastermind who orchestrated the ambush on the police convoy that opened the series and is now setting up her former lover, the admittedly odious DCC Mike Dryden (Mark Bonnar), to take the fall.


Or she could be the innocent party she has claimed to be all along. She has certainly endured a lot: incarceration, beatings, abduction, waterboarding and having her hands deliberately scalded with boiling water by a pair of vicious prison guards.

In the very first episode, her colleagues, including junior ones, ganged up to shove her head down the toilet.

On the other hand, we've seen flashes of her darker, nastier side. When a drunken neighbour refused to turn down her loud music, Denton whacked her across the head with a bottle. At another point, she punched DC Fleming (Vicky McClure) in the stomach.

She has also proved adept at uncovering the investigating detectives' own weaknesses, secrets and lies, and then using them as weapons in her own defence. There's a third possibility: that Denton is innocent of some things but not of others, for reasons which haven't yet become apparent.

One thing we know is that she's a tough cookie and a slippery customer. But then most of the characters in Line of Duty – which for my money is not only vastly superior to the excellent first series but the best thing the BBC has done since State of Play 11 years ago – are slippery, which is what has made it so riveting.

In a way, I'm glad the BBC is playing its cards so close to its chest on this one. With the way the audience has been fragmented, television doesn't offer too many old-fashioned communal experiences any more, unless it's live sport.

Having watched most of the previous five episodes alone at home on a computer, usually early in the morning, it will make a nice change to join the three million-plus viewers in the UK, and however many more are watching in Ireland, all of us biting our nails at the same time.

Ironically, the person doing the most nail-biting will probably be the same one responsible for the chronic state of everyone else's fingers – Mercurio.

He confessed this week that he only decided how Line of Duty should end very late into production and is understandably anxious about how viewers will respond.

The denouement of a drama is usually solidly blocked out from day one. Somehow, Mercurio's admission that he was torn about the fate of his characters almost to the final act adds an extra frisson of excitement and anticipation to the finale.