Episode 4

A serial killer is on the rampage, and the body count is climbing fast. Luther has to set his own troubles aside and get inside the man's head, which is where he is most at home and least comfortable. The question driving the episode is simple to ask and brutal to answer: what is making this man kill so many young women, and why now.

And the pursuit is a strange, quiet one for a show with this much violence in it. Most of the work happens in interviews and second guesses, Luther and the team trying to map a mind that does not want to be mapped. Graham Shand (Rob Jarvis) and his wife Linda (Nicola Walker) sit at the edge of the case, and their presence gives the killer a face before the story is ready to hand him one. Dermot Crowley's DCI Martin Schenk and Michael Smiley's Benny Silver are the new faces in the rotation, while Steven Mackintosh's Ian Reed, Warren Brown's Justin Ripley and Saskia Reeves's Rose Teller hold the investigation together around Luther.

Neil Cross wrote it and Sam Miller directed, and the episode leans on its cast to carry the dread. Idris Elba spends much of the hour in rooms with people who are lying to him, and the performance is all in the listening. Ruth Wilson's Alice Morgan drifts through the edges of the story, a reminder that Luther's private life is not on hold just because the job demands it.

The closer Luther gets to the killer, the clearer it becomes that the motive is not what anyone expected. The episode trusts its audience to keep up with a man whose reasons turn out to be stranger and sadder than a simple monster story.

It ends on the wrong side of a door, with the case solved and nothing about it feeling like a win.