Watertight

The body of Dee, a teenage prostitute, is found murdered, and for Ross it's the match that lights the whole fire. He has long suspected Willie Strachan of being involved in sex trafficking, and this is the case he's been waiting for. The pursuit is personal and it shows. Her age makes it worse, the way it always does in this series.

But the suspects line up quickly, and none of them is tidy. Craig Colhoune, a law student and the adored son of a widowed sheriff, is found near the scene with blood on his sleeve he can't properly explain. Then there's Mo McCleish, violent husband of Sadie McPhail, a reformed prostitute, and a man whose temper makes him a candidate all on his own. Ross pushes hard at Strachan, but the harder he pushes, the more the case seems to push back at him. Before long his career is on the line alongside the investigation, and the lives caught up in the case are on the line too.

The episode was written by Glenn Chandler and Michael Jenner and directed by Brian Kelly. John Michie is DI Robbie Ross, with Blythe Duff's DS Jackie Reid, Alex Norton's DCI Matthew Burke and Colin McCredie's DC Stuart Fraser, and Anne Marie Timoney as DSI Valerie Patterson. Stuart Wilkinson plays Craig Colhoune, Eric Barlow is his father Alex, Garry Sweeney plays Mo McCleish, Rae Hendrie is Sadie McPhail and Stephen McCreadie plays Lyle Farridy.

It's the kind of episode that starts as a straightforward hunt and turns into a test of one detective's judgement. Ross's certainty is the point, and the question the episode leaves hanging is whether it's a strength or a blind spot. The title says it all: everyone involved believes their own story is watertight, and the team's job is to find the leak.