The Knife Trick

Thirty trainee police officers watch a law professor called Michael Shearer demonstrate a knife reconstruction, and watch him accidentally kill another lecturer, Alistair Matheson. Thirty witnesses, one dead man, no obvious culprit. It's a teaching moment that goes wrong in front of the future of the force. It looks like a tragic mistake in front of an audience. Then the team discovers the knife was tampered with, and manslaughter becomes murder.

Matheson had beaten Shearer to a promotion, which gives the demonstration a sharper edge than the syllabus intended. When Matheson's daughter Katie attempts suicide and her encoded diary becomes the kind of evidence people want to see gone, suspicion hardens further. Then another witness comes forward: she saw a suspicious student loitering outside Matheson's office before the demonstration began. The simple accident story collapses, and the inquiry has to start again with thirty possible answers. The trainees were there to learn how a knife killing works, and they got a live one.

Glenn Chandler and Stephen Brady wrote it, Patrick Harkins directed. Andrew Byatt plays Shearer, John P. Arnold is Matheson, William Armstrong is Parkes, Margo Gunn is Pippa and Martin Haddow is Goldie. Around them, Blythe Duff, John Michie, Alex Norton and Colin McCredie keep the investigation moving, with Katrina Bryan's Ellis Sinclair on the forensic side.

But the trick of the title is that a room full of police trainees should be the perfect witness pool, and instead it's the opposite. Everyone saw it, so nobody can be trusted. The episode trusts its audience to spot the irony: a room full of people whose job will be reading crime scenes, watching one get made. It gets real mileage out of that idea, and the killer's identity lands with the quiet satisfaction of a puzzle solved in full view.