1940. Britain is at war. Rector's daughter Wilhelmina Harkness longs to do her duty for her country, but when her strict mother forbids her to enlist, their bitter argument has devasting consequences. Unable to stay in the village she loves, Wilhelmina - reinventing herself as Billie - spends everything she has on a one-way ticket up north. Hull is a distant, dangerous city, but Billie is determined to leave her painful memories behind and start afresh, whatever the cost. The last thing Billie expects on her first evening in Hull, however, is to be caught in the city's first air raid - or to stumble across the body of a young woman, suspiciously untouched by debris. If the air raid didn't kill the glamorous stranger, what did? Billie is determined to get justice, and her persistence earns her an invitation to the newly formed Women's Police Constabulary. But as the case unfolds, putting her at odds with both high-ranking members of the force as well as the victim's powerful family, Billie begins to wonder if she can trust her new friends and colleagues . . . or if someone amongst them is working for the enemy.