Glasgow, 1933. Murder is nothing new in the Depression-era city, especially to war veterans Inspector Jimmy Dreghorn and his partner ‘Bonnie’ Archie McDaid. But the dead man found in a narrowboat on the Forth and Clyde Canal, executed with a single shot to the back of the head, is no ordinary killing.

Violence usually erupts in the heat of the moment – the razor-gangs that stalk the streets settle scores with knives and fists. Firearms suggest something more sinister, especially when the killer strikes again. Meanwhile, other forces are stirring within the city. A suspected IRA cell is at large, embedded within the criminal gangs and attracting the ruthless attention of Special Branch agents from London.

With political and sectarian tensions rising, and the body count mounting, Dreghorn and McDaid pursue an investigation into the dark heart of humanity – where one person's freedom fighter is another's terrorist, and noble ideals are swept away by bloody vengeance.