Every August thousands of tourists and performers flood into Edinburgh for its famous festival. But this year, on the very first day, an explosion rocks Princes Street, and a man is killed. A few hours later, a threatening letter is delivered to the Secretary of State – apparently from a freedom group – demanding political separation from England. Assistant Chief Constable Bob Skinner, head of CID, quickly assembles his team, with SAS backup.

Yet, despite a huge security check, the escalating situation claims its second victim. Skinner begins to suspect that this is no ordinary group of bloodthristy fanatics, but skilled professionals with access to highly sophisticated weapons; a trained task force with an agenda that now threatens the ancient honour of Scotland…