The Indelible Stain

A visiting lecturer at the college of criminology is found strangled after delivering a lecture on controversial theories about genetic predisposition to criminal behaviour. The victim's theories were exactly the kind that make headlines, and the show makes the most of the debate without taking sides in it. The audience that night is full of academics who disagree with him passionately, which gives Lewis and Hathaway no shortage of suspects. The case takes its first sharp turn when one of those suspects is herself murdered.

Her private life, specifically an affair with a married professor of Criminology, sends the investigation in a different direction entirely. The episode moves from a public argument about science to the quieter, messier territory of the affair, and the two strands twist together as the truth emerges. The academic setting provides the polish; the human failure underneath is what the show is really interested in.

But the lecture that started it all was always going to make enemies; the surprise is how quickly the argument turned lethal. Criminology, the episode suggests, is a field where the theories are bloodless and the practitioners are anything but.

Colin Dexter and Simon Block wrote it, and Tim Fywell directed. Harriet Madeley plays Emily Robinson, Richard Southgate is Will Pascoe, David Soul takes Paul Yelland, David Calder plays Andrew Lipton, Pippa Bennett-Warner is Nina Clemens, Jason Durr plays DI Alan Peterson and Clare Holman takes Dr Hobson. Kevin Whately and Laurence Fox lead the investigation.

The affair is handled without prurience, and the married professor's career ends the way such careers do: quietly, with the details left to the imagination.

The episode earns its place by remembering that theories, however forcefully argued, are not the same as behaviour. The lecture hall debates genetic destiny while the people involved make the same old mistakes, and the irony is not lost on the show. It is a reminder that crime, whatever the textbooks say, keeps coming down to individuals.