Scarred by the Troubles in Belfast, Billy Nelson returns to his loyalist roots following his discharge from army service. But an outbreak of gang violence forces Billy to move on to Edinburgh, where he muscles in on the capital's drug trade. As the balance of power tips, underworld rivalries between Edinburgh, Glasgow and Belfast spill onto the streets. With a spate of horrific incidents and a trail of victims, the pressure is mounting for Grace Macallan, new superintendent of the Crime & Counter Terrorism Directorate. Troubled by her own demons and with everyone baying for Billy Nelson's blood, can Grace hold her nerve?