Puppet on a String

A masked attacker with a syringe full of lethal drugs goes for Dr Mellor, and she survives the encounter only to point the finger at Dr Thomas Finn, a professional rival. Finn has an alibi. But it's a good one. The next day Dr Mellor is found dead, and Finn has another cast-iron alibi for that too.

The police are left with a suspect they can't touch and a victim who named him twice. Everyone agrees Finn is the obvious answer, and the evidence keeps waving him through. The attack and the death are only a day apart, and the team are stuck with a man who was provably elsewhere for both. Burke's squad circle the pair, and the investigation keeps returning to the same frustrating place: a doctor who looks guilty as sin and a timetable that clears him every time. The team are used to killers who slip up; this one doesn't. There's no blood trail to follow and no witness who saw anything useful, just a pair of alibis that keep holding.

Julie Dixon and Glenn Chandler wrote it, Morag Fullarton directed. Blythe Duff's Jackie Reid, John Michie's Robbie Ross, Alex Norton's Burke and Colin McCredie's Stuart Fraser handle the case, with Lesley Harcourt's Gemma Kerr in the mix. Jimmy Chisholm plays Dr Thomas Finn, Andrea Hart is Dr Rhoda Mellor, and Paul Doonan's David Tyler, Neil McKinven's Kenny Niven and Louise Beattie's Karen Menzies fill out the supporting cast.

The title suggests someone being worked on strings, and the neatness of Finn's alibis has that same feel of careful arrangement. A solid alibi is the most boring piece of evidence in any murder enquiry, and this episode makes it the star. It's a puzzle built on a single elegant problem, and the team's frustration becomes the viewer's. The mystery is how the killer keeps being somewhere else.