Case Sensitive stars Olivia Williams in the lead role of DS Charlie Zailer and Darren Boyd as DC Simon Waterhouse, who are called in to investigate the deaths of a woman and her child.
Ruth is interviewed by Charlie and Simon. They have a letter from Jason’s house that should have arrived the day that he died with her fingerprints are on it. Ruth admits that she went to the house and she slept with Jason. She insists though that she didn’t kill Jason.
When the bodies of a mother and her young daughter are found in the bathroom of their luxury home, new DS Charlie Zailer and her junior colleague DC Simon Waterhouse investigate. The pair immediately clash, disagreeing over whether the case is a murder-suicide or something even more sinister. Elsewhere, young mother Sally Thorne tells her best friend that she had an affair with the husband of the dead woman, and her friend warns her to stay away.
After a music teacher leaves her husband for a troubled musician, it ends in tragedy. DS Charlie Zailer and DC Simon Waterhouse investigate a case of failed ambition and murderous jealousy, while dealing with their own difficult relationship.
After a mother and daughter are found dead in strange circumstances, a mysterious young woman calling herself Esther visits the crime scene. DS Charlie Zailer and DC Simon Waterhouse track the woman to a bird sanctuary but she disappears, leaving behind only a photograph of an unknown mother and child. Who are they - and what is their connection to the dead family? Then a woman's body is found floating in the river and Sally begins to realise that she is in terrible danger.