As London endures nightly bombing raids, young Ruth and Hatty Miller are doing their bit working as auxiliary firewomen at Good Hope Station in Kentish Town. The girls realise the importance of working as a team, so it's bad news when someone starts sending out poison pen letters. Chief Inspector Jack Stamford is called in to investigate. The obvious suspect is the secretive Nell de Groot, a firewoman with a history of similar behaviour. But when Sergeant Sarah McNeill befriends Nell, she's soon convinced that Nell is innocent. And she's determined to get to the bottom of things quickly, before the malicious writer ruins too many lives already pushed to the limit by war.