In the midst of a rock festival, a charity worker is sliced across the stomach. He dies minutes later. In a crowd of thousands, no one saw his attacker. The following week, the body of a primary school teacher is found in a dumpster in an Edinburgh alley, strangled with her own woollen scarf.

DI Ava Turner and DI Luc Callanach have no motive and no leads - until around the city, graffitied on buildings, words appear describing each victim.

It’s only when they realise the words are appearing before rather than after the murders, that they understand the killer is announcing his next victim ... and the more innocent the better.