Sarah Lancashire returns in the acclaimed BBC thriller written by Sally Wainwright       No-nonsense police sergeant Catherine Cawood is back heading up her team of dedicated police officers in the Calder Valley in West Yorkshire       While on duty, she makes a gruesome discovery – a body       The victim’s injuries bear a striking similarity to a string of other murders over the previous few months, suggesting a serial killer is on the loose       But the case becomes even more shocking when it emerges that Catherine knows the victim – something that could have serious repercussions for both herself and her family