Two familiar faces will appear in the Line of Duty series finale

But who is Sam? If we've learned anything this series, it's that it's always worth swotting up on past events from Line of Duty – they're likely to crop up again.

Sam was the girlfriend of Steve Arnott in the last series. The two were pictured living together, celebrating their one-year anniversary, before Matthew "Dot" Cottan attempted to frame Steve as the "Caddy".

He was remarkably successful in his efforts, managing to get Steve arrested and raising suspicions amongst all his acquaintances – including his girlfriend. It was when Steve was behind bars that Sam decided to dump him, suspecting her other half of being involved in a wide-reaching criminal network.

How wrong she was – but by the time Steve had been released, it was too late to save their doomed relationship. And that was the last we saw of Sam.

What will Steve make of her reappearance in this Sunday's episode? And will her boss ruffle Ted Hastings' feathers?

If you recall, Lester Hargreaves – who heads up murder squad – isn't cut from the same cloth as AC-12's scrupulous boss. His approach is somewhat more lax when it comes to investigating criminals. But you may notice one crucial detail about his name: Hargreaves begins with an H.

Dot's dying declaration revealed there was an officer, whose name begins with "H", embedded within the senior ranks of the police force and working for the mysterious criminal network AC-12 are investigating. So far we've suspected ACC Hilton, DCI Roz Huntley and Ted Hastings (God forbid) but could Hargreaves be the person we're looking for?

Guess we'll have to watch Sunday's episode to find out.