Idris Elba is slipping back into that big overcoat for another turn as the former DCI John Luther in Netflix’s Luther: The Fallen Sun, and there’s a lot to catch up on if you’ve been putting off diving into the bloody underworld he tends to kick about in.

Last time we saw him he was being sent down himself, having blurred the lines between detective and ruthless avenger just a touch too often. But how did we get there? This is the essential primer for Luther: The Fallen Sun.

Who is Luther?

OK, back to square one: DCI John Luther is a detective who hunts down the most dangerous and psychologically complex killers London has to offer. He’s a widower, and he’s a grump, and he’s absolutely not averse to cutting deals with criminals if it helps him to get closer to taking down the evildoer he’s become obsessed with most recently.

Obsession is a bit of a theme with John Luther, as is taking retribution into his own hands. Kidnap, blackmail, beating people up, covering up crimes: all fair game.

“Luther, he’s probably the character that’s most like me, in real life,” Elba told us in his recent cover interview. “He has so much conviction. He doesn’t try to overcomplicate things. He gets to the point.”

What’s happened so far in Luther?

We’re five series in at this point, so we might have to do some condensing here. Basically: he was on the trail of a murderer called Alice Morgan for a bit, then they became mates and had a bit of a flirty thing going on, then she apparently died. But! Then she came back. Then she apparently died again.

To be honest, this is probably about as much as you’ll need to get on with Luther: The Fallen Sun. Director Jamie Payne says you don’t need to know that much about Luther to get into the film.

“We thought it was important that if someone was watching the film for the first time that it had its own story,” he told Entertainment Weekly. “So you could watch the film and go back and then binge the series. It was important to all of us that the audience could find a way into the series, and to the lore.”

Perhaps the most important returning character to know is DSU Martin Schenk, played by Dermot Crowley. He used to be Luther’s commanding officer and has a hatred of bent coppers to rival Ted Hastings from Line of Duty. He and Luther haven’t always seen eye to eye but you suspect he might end up being important in helping Luther out of his brush with the law.

Where did we leave Luther at the end of the last series?

Of late, Luther’s been spending time at His Majesty’s pleasure, after bending the law one too many times. He’d managed to foil a hitman who’d been sent to kill him, but who then fed the police information which suggested Luther had beaten up the man who’d tried to have Luther killed, George Cornelius.

Alice had decided to kill herself by jumping off a platform at a building site, and we saw her body lying apparently lifeless at the bottom. But then, like Michael Myers at the end of Halloween, she was gone.

Is Ruth Wilson’s Alice coming back?

She’s not in the cast list which has been sent out ahead of the release of Luther: The Fallen Sun, but given that she’s already died once and come back it’s not beyond the bounds of possibility.

“I don’t know how someone could survive that fall, so that’s my thought. But then you don’t see the body,” Wilson said after the climax of the last series. “So I don’t know.” Cheers Ruth!

What’s going to happen in Luther: The Fallen Sun?

There’s a serial killer on the loose in old London town and Luther busts himself out of prison to sort out exactly who it is before they strike again. Exactly who it is, is Andy Serkis. He plays a big bad tech billionaire – lots of those about at the minute – who’s using his big bad tech to surveil and murder. Meanwhile, Cynthia Erivo’s Odette Raine is a counterintelligence op, who’s on the trail of Luther after his break-out, and is determined to bring him to justice.

It looks like we won’t be sticking to London this time. There are images of Luther stomping across a snowy mountain in his big coat, walking down a creepy wooden corridor in his big coat, and looking a bit shocked in his big coat – but, crucially, not his tie. Intriguing.

Are we looking at a Luther film franchise?

Possibly. “Hopefully this is a series of movies expanding the John Luther stories, bigger canvas, more bandwidth to find out who he is, understand him a bit more,” Elba told Esquire.

‘Luther: The Fallen Sun’ is in select cinemas from 24 February, and hits Netflix on 10 March