Time to Murder and Create

The fourth series begins with Tony Hill's authority in pieces, which is a braver opening than a new partnership deserves. A rapist convicted years earlier on the strength of Hill's first forensic psychological profile has been released on appeal, and the man who built the case against him is discredited with him. Hill's therapist thinks he is resisting treatment because he clings to the dark side of his own psyche. Carol Jordan will not return his calls.

She has also gone. Hill discovers that Jordan has left the Bradfield force for South Africa without telling him, and her replacement, DI Alex Fielding, is young, female, friendly and clear that she does not want him anywhere near her work. The show handles the recasting by report rather than by scene, which is the most criticised decision of the run: an audience that had followed the Jordan partnership for three series is told, not shown, that it is over.

What follows is a good case built on a familiar idea. Fielding assumes a body found in a field is the dead wife of a convicted matricidal farmer. Then a second body appears, and a pattern of missing young women emerges under similar circumstances, which forces her to accept that Hill's serial-killer reading is right. The killing method is where the episode is most of its time: the women are being lured through a sadomasochistic internet site, their own fantasies used against them until the killer tires and moves on.

That is a bleaker premise than the show had used before, because it makes the victims participants in their own danger — and Patrick Harbinson's script, wisely, does not moralise about it. The point is that the fantasies are ordinary, the by-products of unremarkable lives, and the killer's skill is in recognising which ones can be exploited. It is the closest the series comes to a portrait of predation as patience.

Simone Lahbib has the hardest job in the series: replacing a beloved character without imitating her. She plays Fielding as competent, direct and unimpressed, and the show gives her the resentment rather than the warmth, which is the right choice. The partnership is rebuilt from professional hostility, and the audience is allowed to watch it happen rather than being told it has.

The weaknesses are structural. Jordan's departure happens off screen and without explanation, so the emotional weight of the change is carried entirely by Green, who has to play bereavement in a show that has no room for it. And the episode carries a heavy load of reset in its first third, which makes the mystery slower to start than it should be.

It drew 6.51 million viewers, a drop from the third series. It is the hinge episode of the entire run, and its importance is hard to overstate: everything the show does from here depends on the audience accepting a new partner, and it earns that acceptance the hard way, by making her difficult.