The ninth series sees Detective Superintendent Boyd and his cold case team tackle their most challenging and personal cases to date. Foisted upon Boyd by the top brass of the Police Force is a new team member: DSI Sarah Cavendish, a brilliant counter-terrorism officer who matches Boyd in rank. He knows she must have fallen from grace to end up with the Cold Case squad alongside him - but how? As the team pick up on Sarah's on-going symptoms of post-traumatic stress, Sarah repeatedly challenges Boyd's unorthodox take on police procedure - something, she warns, he will pay for one day...