Wealthy trader and serial killer David Robey (Andy Serkis) blackmails and kidnaps cleaner Callum Aldrich. DCI John Luther (Idris Elba) is assigned to the case and promises Callum's mother, Corinne (Hattie Morahan), he will find her son. Concerned about his involvement, Robey digs up dirt on the various illegal acts Luther has committed as a police officer (he comes across as a vigilante who takes law into his own hands and feels that the rules don't apply to him), resulting in him being fired, prosecuted and imprisoned.Years later, Robey coaxes Corrine and the parents of other victims to a house (he had kept them alive all these years) where he burns their children's corpses (the parents finds their kids hanging in the basement when they arrive at the house. A overheated mobile phone battery triggers a fire). The house belonged to a Saudi national, who hadn't set foot in Britain for several years. Corrine visits Luther in prison, admonishing him for not finding her son's killer. Robey sends a recording of him murdering Callum to Luther in prison via a radio frequency (which was passed on to Luther in prison, via a note under his door). In a twist, it was Robey who drove Corrine to prison, as nobody still knows that Robey is behind the kidnappings and the murders. Robey has positioned himself as sympathetic to all the parents who lost their kids to the serial killer. Luther informs DCI Odette Raine (Cynthia Erivo) over phone, the new head of Serious and Serial Crime, of the broadcast. Raine tells him that the killer sent the recordings to the families of all the 8 victims. Luther knows something big is coming and the killer is looking for an audience. Luther wants to be one step ahead of the killer. Raine tells Luther that she doesn't care about a dirty cops unfinished business and refuses to help him in any way. He liaises with prison guards and former associate McCabe (Vincent Regan) to break him out of prison. Luther bribes the guards to let the other prisoners attack him (they hate Luther as he was a cop). This then becomes an excuse for the guards to transfer Luther to another prison, and for McCabe to break him out, en-route.Raine brings in retired DSU Martin Schenk (Dermot Crowley) as a consultant to help track down Luther. Schenk used to be Luther's boss. Raine promises to kill Luther, if he messes with her investigation. Luther finds the radio station that is broadcasting Robey's radio message. From the owner of the radio station Derek, he learns that the owner met a man online, who was into extreme submission porn. Derek was give an address and security codes for a meeting. Derek reached the place and found a man waiting for him, and proceeded to have sex with him. Derek later realized that this wasn't the man he met online, and that he had just raped a complete stranger. He then received blackmail threats and was told to play the broadcast tape without asking any questions. Luther takes the phone on which Derek is contacted. Luther contacts SChenk and asks him to trace the number. Schenk keeps Raine in the loop.Luther traces Robey to Piccadilly Circus, where Raine deploys armed police SCO19 (since they both got their info from Schenk). The two confront Robey there (Luther calls the Killers phone and watches a man in a parka pick up), but some people Robey has blackmailed cause a distraction by killing themselves jumping from buildings and crashing cars. Robey escapes into the London Underground after fighting Luther and murdering an armed policeman (he cuts the man's artery in the leg, which causes him to bleed to death while Luther tries to save him). An officer on Raine's team, Archie, is shown to be in cahoots with Robey.Luther speaks to Raine on the phone and speculates that the killer blackmails his victims to do his bidding. Luther meets Corrine and tells her that Callum was perhaps blackmailed too. He speculates that she has met the killer and that he is in her life. Corrine talks about a man named Tommy who met her at a support group after Callum's death. Robey kidnaps Raine's daughter, Anya (Lauryn Ajufo). By now Raine knows the killer's real name. Robey wants Luther dead in exchange for Anya. Raine meets with Luther and reluctantly agrees to work with him. She tells Luther that Robey was suspected of sexual assault 6 yrs ago, but it was never proven. Then his wife was about to leave him, when she was caught in a fire, and horribly scarred, but survived. Again, arson was never proven. Raine tells Luther that Robey has her daughter and she has to deliver Luther to him. Meanwhile Raine's team figures out that a "red Room" is being organized on the web. The members get to chose how the victims die. The victims profiles match the people who disappeared all over Europe in the last 6-8 weeks.They visit Robey's ex-wife Georgette (Tara Fitzgerald) (the one who was disfigured in the fire. Luther says he knows Georgette tried to stop Robey which is why he set her on fire), discovering he owns property abroad (She agrees to help after Raine says that Robey had Anya). Archie had set a alert on Raine's passport and informs Robey that she has crossed over into mainland Europe. Robey knows that only Georgette could have directed Raine to Europe. DS Archie Woodward (Thomas Coombes), Raine's subordinate, is blackmailed by Robey into killing Georgette but is intercepted by Schenk (who was sent by Luther to protect Georgette). Archie commits suicide.Luther and Raine travel to Robey's mansion in rural Norway, where they discover he tortures kidnapped victims on live-stream called "The Red Bunker" for his followers. The two are overpowered, and Robey tries to force them to hurt each other to save Anya. Luther reveals Georgette told the police the location and they are en route. After a brutal fight, the three are able to escape the bunker, and Luther chases Robey into a frozen lake, where Robey drowns. Luther is rescued by police divers and Schenk. Recovering from his injuries back in London he is approached by Cranfield (Guy Williams), a senior official with MI5, who, it is implied, offers him a job in lieu of going back to prison.