The Smile of the Stranger — Joan Aiken

He looked so like her hero, Charles the First, that it was easy to fall in love -- and into trouble.

She shopped along the Ponte Vecchio. She spoke Italian and French with fluency and knew more English history than many a girl born and reared in England.

But she had grown up in Florence, the daughter of a self-exiled Englishman who had fled to Italy when his dreams disintegrated. It was scarcely an upbringing that would prepare Juliana to outwit a French mob, escape in a balloon across the channel and match wits with the suave gentleman who was the Prince Regent's agent. But she managed it all until she met the experience she was least prepared for -- when a man who looked like her dream hero begged to be her love.