Isabelle of Paris

The principal character is 15-year-old Isabelle Laustin, other characters being her parents Leon and Marie, Isabelle's friends Jean and his sister Marie, and Isabelle's suitor, Captain Victor.

When Paris is besieged and Napoleon III's army is defeated, Isabelle escapes to London, disguised as a boy, and engages in secret agent stuff.

(Source: AniDB)