Episode 6 (S04E06)

As the season comes to a close, Jimmy grapples with unsettling DNA evidence that suggests Duncan could be implicated in both Lizzie Kilmuir's murder and possibly Sally McColl's. When he brings a bewildered Duncan in for questioning, the latter is deeply offended that Perez would suspect him of such a crime. Yet, Perez is convinced that Duncan is hiding something important. Eventually, Duncan confesses to having had an affair with Donna, but just when it seems the truth is out, a shocking revelation emerges that changes everything.