It’s 1914 and Harriet is a girl who doesn’t quite fit in. When she’s asked to leave the Girl Guides for fighting, it turns out to be bad timing. War breaks out and the efforts of Scouts and Guides are volunteered in support of the war effort; Harriet feels side-lined. A local family, the Browns, are revealed to have German connections, but after Thomas Brown heroically saves Harriet’s younger brother on the railway track, Harriet and Thomas become firm friends.

Harriet decides to form her own ‘army’ of misfit children, mounting patrols and searching for German spies. It’s all a bit of a joke at first, and their first ‘catch’ turns out to be the new Welsh postman. Harriet joins her friend Violet to wave Violet’s brother Stephen off as he leaves for war, alongside his mates. The campaign against the Browns grows nasty: Mrs Brown loses her job, and Thomas’s younger brother Lewis is attacked in the street. Harriet tells Thomas that she has a plan to help them.

Visitors to BBC Learning’s World War One At Home Tour this summer can take part in Harriet's Spy Quest. Children visiting events in Bristol, Rhyl, Portrush and Nottingham can have a go at code-breaking and find out about espionage with an outdoor quest.