Burgundy 1728. Old count Antoine d'Eon is overjoyed. His daughter-in-law has finally given birth to a boy and he will at long last be able to transmit his inheritance to his son Pascal. At least this is what he thinks, for Pascal has concealed from his father the fact that his son was ... a daughter, his eighth daughter! Well, enough is enough and Geneviève has no other choice but become a boy first, then a soldier and even the special envoy of Louis XV, King of France, to Catherine II, the Czarina of Russia.