It got great reviews, featured brilliant performances and kept viewers hooked to their screens for five weeks - and then vanished without a trace, seemingly forever.
But, almost two years on, Line of Duty is making an eagerly awaited return to our screens - so it isn’t a TV one-hit wonder after all.
For those who need a reminder, the first series introduced straight-arrow anti-corruption cop Steve Arnott and his colleagues, who were investigating the possible crimes and misdemeanours carried out by DCI Tony Gates.
Martin Compston, Vicky McClure and Adrian Dunbar are among those returning to the drama, while Call the Midwife’s Jessica Raine joins the cast as a rookie detective constable.
Sadly Lennie James, who played Gates, isn’t in this series because Arnott and his team will be tackling an investigation into a different case of suspected police corruption.
Series writer and creator Jed Mercurio says of the focus of the series’ investigation: “I wanted to create a character as far from Tony Gates as possible.
“Detective Inspector Lindsay Denton appears to be a mild-mannered, backroom detective _ and her social and professional isolation make her a figure of mistrust. Playing against type, Keeley Hawes is an absolute revelation.”
Denton is the sole survivor of a police convoy ambushed while escorting a protected witness. Arnott immediately suspects she tipped the perpetrators off - but proving it may be difficult.
“At first when I went and met Jed and Douglas, the director, they weren’t too sure whether Lindsay would be on the dark side or not, which was so fascinating for me,” says Hawes. “I was sent the scripts and I just sat there and read them one after another in one sitting. I still had no idea and I begged them to tell me - regardless of whether I got the part I just needed to know!
“They told me that they hadn’t decided so it was quite incredible to go in and play the scenes completely not knowing whether I was guilty or otherwise. I’ve never had to do anything like this before.”
There was one aspect of playing Denton that Hawes wasn’t so enthusiastic about, however.
“I didn’t love going into the make-up truck every day and having them look at me saying ‘yep, that’s great!’ without even brushing my hair, actually only putting a bit more dark under my eyes!
“But it’s quite liberating once you get used to not even bothering to wash your hair and people just adding a bit more grease. There is one point where she has really been through the mill - by the end of that scenario, I think nobody has ever looked that bad on the television screen. It’s liberating for me as I have played lots of glamorous characters and this is the absolute opposite.”
But is Denton guilty or not? Hawes won’t spill the beans, and thank goodness for that - this is one series we’re looking forward to watching unfold for ourselves.