I USED to wake up each Thursday with a spring in my step and a song in my heart because that was the day Line of Duty (BBC1, Sunday) would be released to me. I’d send my little email to the publicist, receive a list of spoilers which I must agree to keep secret, then I’d hit “play” and watch this brilliant show unspool its twists, turns and revelations.

Then I’d have to spend the rest of the week hopping and fidgeting, and having to clap my hand over my mouth in case I blurted out the brilliant plotlines.

I’d be dying for Sunday night to arrive so I could dive into the morass on Twitter and, finally, discuss the show and see what wild theories other viewers had. Oh what a day Thursday was!

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But no longer. Thursday is just a day like any other now.

The alarm goes off and I grumble and turn over. The dog nudges at my elbow, wanting pats and pee-pees. The sun comes up and I rub my eyes and wonder what’s on the to-do list for today …must get a copy of Line of Duty …yeah, big deal. I turn over and go back to sleep.

I loved every series, every single episode of this show, until now when, in its grand old age, I think it’s crumbling under its own weight.

Previously, the storylines were shocking but could always be pinned to reality. Now reality has been chucked out the window in this fourth and final series.

The writer is going purely for shock and twists, and has forgotten the fine old rule that you can get away with anything as long as it’s still plausible.

Let us follow the line into the madness, but be sure to make the line tough enough that we can find our way back out into the light when it’s over.

We’ll gladly go with you into shootings, car chases and the bleakest evil as long as you can, as the teacher used to say, show your workings.

In this silly series, writer Jed Mercurio has not only omitted to show his workings, but he’s using a calculator, forgetting that the viewer is still using a pencil and jotter and is scratching his head and saying: “What? How did you get that answer?”

When the viewers do their sums, and catch up, they find they have a different result from Mercurio, so I think we’re beginning to suspect he’s doing his sums on a dodgy calculator he got on a stall at the Barras. His wild sums are no longer adding up. All his arithmetic worked out beautifully in every other series.

Even when it all seemed tangled and hopeless, he managed to bring everything to a gorgeous conclusion. His sums worked out and we saw how he’d brought us there, but in this series crucial numbers have been erased, the plus sign has been turned to a minus, and he has forgotten to carry his remainders. The whole thing is a jumbled mess. Firstly, he upset the perfect chemistry of AC-12. The hardy trio have been split up in this series, with Fleming and Arnott hardly sharing any screen time, and a new boy brought in.

As for Roz, her inscrutable face initially suggested she was bravely hiding a rising panic which was promising to explode, but now her inscrutability just seems dull and each fresh piece of bad news is met with the same blank expression. The “baddies” in previous series were similarly frosty but had an unpredictability that kept us on our toes. Remember the cold and stern Lindsay Denton when she suddenly bashed her neighbour’s head in with a wine bottle?

Roz is at the centre of this story but she is a blank. If she is indeed going to crack and send flame and terror into the hearts of all around her then she’d better get a move on, because we’re halfway through the series and I’m becoming bored.

In fact, the only emotion she provokes is annoyance when she does something utterly implausible and silly – although that’s not her character’s fault, it’s down to the writer. Consider when she hid red-hot evidence in the staff locker or in the boot of her car just outside. So, with the central trio dissipated, the main character a bland, emotionless woman, and the plot tediously tangled with nothing pinning it to reality, I have almost lost hope in Line of Duty.

I can hardly believe I’m saying that.

I’D like to say something good about the telly but it’s been a bad week. Peter Kay’s Car Share (BBC1, Tuesday) resumed and is so light and gentle that it could be issued to those in recovery in a neurological ward.

Broadchurch (STV, Monday) remains compelling, if a bit daft, with every man being made to look like a mean ole cowboy in a mean ole town. They need a twanging guitar soundtrack and swinging saloon doors, and David Tennant's Detective Inspector Hardy should spit on the dusty ground whenever he meets a suspect – although, being Broadchurch, he’d meet them at ice-cream parlours, jaunty chip shops and fairylit whelk stalls.

Next week’s TV is much better, I can assure you, bringing us new comedy and the KGB.