Muriel Gerard returns from church with her mother one Sunday morning to find that her husband, Andrew - a former DCS from Scotland Yard, has been shot dead. She informs Nick who arrives with Kate who confirms the death. A huge murder hunt then starts and two detectives from the Met - a DCI Merton and DC Langdon are sent up from London to take charge of the investigation. Langdon whose an old friend of Nick's stays with him and Kate at the police house. Its thought that Gerard was killed by an old criminal who he'd put away who was now free and wanting their revenge. But after questioning from Langdon suspicion falls on his wife -Muriel. Nick hears from Alex via Kate that Gerard was known for beating Muriel and so it's thought that she just killed him in anger for her pain and misery over the years. However when she is arrested and taken to the station Nick realizes that Muriel's mother - Victoria Wainwright is in fact the murderer and goes to see her and she confesses to it. She couldn (thetvdb.com)