Wealthy city trader and serial killer David Robey blackmails and kidnaps young cleaner Callum Aldrich. DCI John Luther is assigned to the case and promises Callum's mother, Corrine, 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, resulting in him being fired, prosecuted and imprisoned.Robey coaxes Corrine and the parents of other victims to a house where he burns their children's corpses. She 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 an FM radio frequency. Luther informs DCI Odette Raine, the new head of Serious and Serial Crime, of the broadcast. He then liaises with corrupt guards and former associate McCabe to break him out of prison. Raine brings in now retired DSU Martin Schenk as a consultant. Luther traces Robey to Piccadilly Circus, where Raine deploys SCO19. The two confront him there, but people he 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 AFO.Robey kidnaps Raines' daughter, Anya. Raine meets with Luther and reluctantly agrees to work with him. They visit Robey's ex-wife Georgette, discovering he owns property abroad. DS Archie Woodward, Raines subordinate, is blackmailed by Robey into killing Georgette, but is intercepted by Schenk. He commits suicide via air embolism. Luther and Raine travel to Robey's mansion in rural Estonia, where they discover he tortures kidnapped victims on livestream 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 the psychopath drowns. Luther is rescued last minute by police divers and Schenk. Recovering from his injuries back in London he is approached by Cranfield, a senior civil servant with MI5, who are implied to offer him a job in lieu of going back to prison.