Unknown to DI Luc Callanach and the newly promoted DCI Ava Turner, a serial killer has Edinburgh firmly in his grip. The killer is taking his victims in the coldest, most calculating way possible – engineering slow and painful deaths by poison, with his victims entirely unaware of the drugs flooding their bloodstream until it's too late. Faced with their most difficult case yet, Callanach and Turner soon realise they face a seemingly impossible task...