This... is not your typical mystery. I'm not sure it's actually a mystery at all. Poirot is in it, of course, being his usual smug self, but honestly you could swap him out for James Bond and the story would be no different. It's all evil masterminds in secret mountain lairs, with gas bombs and faked death and kidnappings and someone actually tries to kill Poirot by dropping a tree on him and really, I think dear old Agatha must have been experimenting with her smoking material because the whole thing is just batshit insane in the thriller model. Fun, but insane, and all that insanity piled on top of itself tends to distract, I think, from the fact that the plot is not all that well hung together. It seems to rely on a series of coincidences and off-screen shenanigans, and it's nowhere near as complete and as intelligent as some of Christie's best works. Entertaining, though.