It's gonna be a prequel. The screenplay is done and all of the major players have signed on. The final scene of the movie is going to be the first scene of the show's first episode (Luther chasing Madsen into the abandoned factory).
‘Luther’ Prequel Film Written, Says Creator
Luther creator Neil Cross confirmed prequel movie plans for the BBC series starring Idris Elba, saying he’d finish a screenplay and hopes to film next year. The film will follow Luther’s early career when he is still married to Zoe; the film’s final scene is the opening scene of the TV series first season, Cross said at the Edinburgh TV Festival, according to multiple press reports. “Idris is a brilliant leading man, and we’ve hoped to turn Luther into a movie for a long time,” Cross said.
Related: WME Signs Idris Elba
Elba is on board to play John Luther, as are Warren Brown, Indira Varma and Steve Mackintosh. Cross said he went with a prequel, rather than a follow-up to the series, so he could bring back popular supporting characters — including some who let’s say maybe don’t fare so well in the final season. Ruth Wilson’s Alice Morgan character, who doesn’t fit in, chronologically, with the prequel plans, is being mulled as a spinoff series, Cross told Variety last year.
Shame that Alice Morgan won't be featured. She's one of the highlights of the show and I'm curious how her relationship with Luther evolved after the 3rd series. She might get her own standalone show though.