The Great Impersonation by E. Phillips Oppenheim

There was something dramatic, in the most lurid sense of the word, about the brief telephone message which Dominey received, not so many hours later, from Carlton House Terrace. In a few minutes he was moving through the streets, still familiar yet already curiously changed.

Men and women were going about their business as usual, but an air of stupefaction was everywhere apparent. Practically every loiterer was studying a newspaper, every chance acquaintance had stopped to confer with his fellows. War, alternately the joke and bogey of the conversationalist, stretched her grey hands over the sunlit city. Even the lightest-hearted felt a thrill of apprehension at the thought of the horrors that were to come. In a day or two all this was to be changed.