This came out in 1909, the same year as another quick Spy Thriller - Spies of the Kaiser, by William Le Queux. I enjoyed Le Queux, but hanging with Futrelle's creation, Isabel, is a lot more fun. Her nemesis is Special Agent Grimm. At stake is the fate of the world, and the threat of world war, if certain countries secretly sign a pact to crush the power of the USA, and Britain, with methods that would include a new super-weapon. Isabel can actually be labeled the "enemy agent" - the baddie - with Mr. Grimm getting snarled up in her strange and strangely connected adventures. Thefts, kidnappings, as well as dastardly deeds that more obviously link to the main, world-shaking, plot - it all gets explained in the end. Before that, it's murky...and complicated by love blossoming between the two combatants. It's easy to cheer for Grimm, while falling for Isabel...and wondering what her true motivations are.
The pieces end up fitting in a slightly contrived manner, so all the exciting set-pieces and unusual occurrences fit a pattern, and all is explained by the finale. Where will falling in love fit in, as the world's fate is decided?...I won't tell.
This seems to me to be just as worthy an early Espionage Thriller to read as several more famous books it pre-dates: The Great Impersonation by E. Phillips Oppenheim; Bulldog Drummond by 'Sapper'; The Haunted Bookshop by Christopher Morley; Maugham's Ashenden tales; anything by Arthur B. Reeve or Sydney Horler (more Crime & Mystery, than Espionage, in the case of Reeve's The Adventuress and so forth, but with a similar feel). This is like watching Fritz Lang's silent fim of the 1920s, Spies, and seeing a lot of the groundwork laid for more famous action-oriented or 'glamorized' spy shenanigans to come, like James Bond.