D.I. Will Wagstaffe and his team investigate the mysterious death of a young nurse on the London Underground. Whilst CCTV reveals the victim fleeing an invisible assailant, forensics find a DNA link to a missing teenager presumed dead for seven years. Aided by his new team D.S. Rob Mullan, D.C. Ray Monk, D.C. Annie Webb and pathologist Dr. Luke Paul, Will unearths a bond of secrets and betrayals which leads him straight into the dark underbelly of the city.