Mary Ann Cotton is not just the first but perhaps the UK's most prolific female serial killer, with more victims than Myra Hindley, Rosemary West, Beverly Allit or male predators such as Jack the Ripper and Dennis Nilsen. But her own north east of England (and criminologists) apart, she remains largely forgotten, despite poisoning to death up to 21 victims in Britain's 'arsenic century'. Exploding myths that every serial killer is a 'monster', the author draws attention to Cotton's charms, allure, capability, skill and ambition -- drawing parallels or contrasting the methods and lifestyles of other serial killers from Victorian to modern times. He also shows how events cannot be separated from their social context -- here the industrial revolution, growing mobility, women's emancipation and greater assertiveness. And concerning the reticence of 'human nature', like Dr Harold Shipman, Cotton was allowed to go on killing despite reasons to suspect her. The book contains other resonances to aid understanding of how serial murderers can go undiscovered despite such things as coincidence, gossip, whispers or motives that become more obvious with the benefit of hindsight. It is also a detective story in which the persistence of a single individual saw Cotton tried and executed, events analysed first-hand from the archives and location visits as the author fills the gaps in a remarkable story.