DI Helen Birch is recovering from major surgery, housebound and exceptionally bored. Her boss, DCI McLeod, has made it crystal clear: she is not to take on any work until her recuperation is over. In her absence, Amy Kato is promoted to sergeant and is given a maddening case to work on: Edinburgh is being plagued by an anonymous vigilante. He started small, meting out punishment to obnoxious boy racers and other antisocial folk, but his behaviour is escalating. Amy can tell from the anonymous online paper trail he leaves. His writings are increasingly confident, and increasingly threatening. Birch finds it impossible to resist helping - placing her directly in the path of immense danger.