Told across a stifling week during an August heat wave in London the 1 x 120 minute episode of Dark Heart follows DI Wagstaffe and his team investigate what unfolds to be not one but a string of horrifyingly brutal murders the victims of which are men who have been accused of sexual offences against children but have never been convicted for their alleged crimes. Staffe feels sure he knows what is driving the killer but finding a connection between the murder victims or even those abused by the murder victims frustratingly eludes him. As dogged and determined as ever Staffe pushes the boundaries in searching for the killer but the truth he finally uncovers is more surprising and shocking than he could ever have imagined.