In mid-December, a fifteen-year-old girl is found dead on a train line near Derry, and Detective Sergeant Lucy Black is called to identify the body. The only clues to the dead teenager's last movements are stored in her mobile phone and on social media - and it soon becomes clear that her 'friends' were not as trustworthy as she thought. Lucy is no stranger to death: she is still haunted by the memory of the child she failed to save, and the killer she failed to put behind bars. And with a new boss scrutinizing her every move, she is determined that - this time - she will leave no margin for error.