An Eye for an Eye by William le Queux

"Remain patient and I'll explain," Eva answered, glancing at the new-comers. "First, however, let me relate a very curious circumstance. Hartmann, who lived somewhere in London, we saw seldom, but very soon after taking possession of The Hollies, there one day called an old friend of Madame's, accompanied by her husband. They were the Blains. Mrs. Blain afterwards came frequently to us at The Hollies, and we often spent the day at Riverdene, while so intimate did the two women become that Madame took the house next to that rented by Mrs. Blain in Kensington."

"Next door? " I gasped, astounded. "In Upper Phillimore Place?"