Did you know that the Girl Guides worked undercover for MI5 during the First World War? Eleven-year-old Annie and her older sister Phyllis join the Guides hoping for adventure, despite their parents' fears that the Guides are unladylike. Joining soon after the start of the First World War, they're swept up in a bigger adventure than they'd bargained for when Phyllis is recruited as a messenger for MI5. Annie, meanwhile, is off investigating a mystery of her own, suspecting the family who run the local haberdashery shop of being German spies. When one of the officers at M15, the stern Captain Warren, goes missing, the sisters find themselves joining forces. Could he be the real spy? And who is behind his suspicious disappearance?.