The bloated remains of a man are discovered bound to a derelict pier in Orkney and newly promoted DCI Lukas Mahler dispatches a team to investigate. But when the body is identified as Alex Fleming - Mahler's former colleague from his time in the Met - the case becomes personal. Mahler's investigation takes him from his old stamping ground of London to the world of organised crime, and from sixteenth-century witch executions to Fleming's most notorious unsolved case: the 'Witchfinder' murders. Are the runic symbols found with Fleming's body proof the killer's struck again - or is there an even darker story to be uncovered? With pressure mounting from all sides and demons from his own past surfacing, Mahler is faced with the most complex moral decision of his career.