The Day of Temptation by William le Queux

Before she had parted from the Marquis she had made a demand boldly and fearlessly, to which, not without the most vehement protest, he had been compelled to accede. She knew him well, and was aware that, in order to gain his own ends, he would betray and denounce his nearest relative; that, although a shrewd, clever statesman, he had won universal popularity and esteem in Italy by reason of certain shady transactions by which he had posed as the saviour of his country. The revelations she could make regarding the undercurrent of affairs in Rome would astound Europe. For that reason he had been forced to grant her what she asked in return for the incriminating paper from the archives of the Embassy.

For over an hour they sat together in the darkness engaged in a strange discussion, when at last they rose and together walked on, still deep in conversation. The Marquis had an appointment, and was about to take leave of her when, as they crossed the wide deserted space between the Admiralty and the Horse Guards, a man in a heavy fur-trimmed overcoat and felt hat, in hurrying past, gazed full into the faces of both. At that moment they were beneath one of the lamps flickering in the gusty wind, and he had full view of them.