As Walter Berrisford paints beautiful Katherine Garland, she asks him to put a ladybird on her finger without his knowing why. He is appalled when he discovers that Katherine is a Nazi. The outbreak of war means that her sister Caro and her friend Robyn join the FANYs, while former maids, Betty and Trixie, work in a factory. War brings frantic romance to all, including their flatmate Edwina O'Brien, but it is Betty, transferred to decode at the Park that alone discovers the truth about the Ladybird.