Lady in the Briars (Black Sheep #2)1990

Rebecca Nuthall, downtrodden unpaid companion to a relative, is saved from drowning by Lord John Danville. Lord John has been ordered abroad by his father after indulging in a frivolous but near fatal duel. About to leave for Russia with his cousin Teresa Graylin and her diplomat husband, he persuades Teresa to take Rebecca along as governess for their little girl. In St. Petersburg, Rebecca blossoms…until she is arrested for espionage. Once again, John must risk his life to rescue her.