Marwood makes his vendetta personal - targeting Luther and those closest to him, and raining destruction on everything he touches. Stark and Gray arrive just in time to hurry Mary to a secret place, and then arrest Luther for murder. But with Gray's unease growing and Luther's escape from custody, is Mary's safety really Stark's priority, or is she just a pawn in his game to lure Luther into a confrontation?

In what looks like a gangland crime - two hoodies are shot at close range while they were in the process of attacking a young couple. But when the team discover they have a vigilante killer at the beginning of a crusade walking the streets, Luther must confront his darkest demons in order to bring him to justice. Meanwhile, Gray and Stark visit Luther's new girlfriend Mary with a warning.

Luther's date with Mary, and the promise of romance, is interrupted by the news of another murder. The killer has struck again - the number of victims is escalating fast. With his working relationship with Ripley rapidly deteriorating, Luther trawls through cold case files, determined to predict the killer's next move. Meanwhile, Erin Gray makes it clear to Luther that she is still pursuing him.

DCI John Luther returns to investigate a twisted fetishist who is murdering women in a horrific echo of an unsolved case from the 1980s. Meanwhile, Ripley is introduced by embittered Erin Gray to creepy, "unretired" DSU George Stark, who is obsessed with proving that Luther has been taking the law into his own hands. Unaware of the looming threat, Luther considers a new romance with Mary Day after they literally bump into each other in the street.