Unknown, alone, and fearing for your life. As PC Lucy Clayburn is about to find out exactly how this feels, going undercover is the most dangerous work there is. But, on the trail of a prolific female serial killer, there's no other option – and these murders are as brutal as they come. Lucy must step into the line of fire – a stranger in a criminal underworld that butchers anyone who crosses the line. And unknown to Lucy, she's already treading it...