Case Sensitive 1x2

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.