The basic plot of this novel involves a fairly stupid young man who ends up shuttling back and forth between the UK and Prague, engaged in what he's been told is mild industrial espionage, but turns out to be rather more serious spy business. There's the same shift from light comedy to mortal danger that we saw last time with Ambler, though this time more abrupt; there's again the shadow of Greene ("The Third Man" gets an oblique nod).
Now, It must be quite hard to write sympathetic stupid characters. Easy enough to write total chuckleheads, I suppose, characters to laugh at; hard to write someone who's daft enough to make the wheels of the plot turn, but appealing enough for the reader to hope the wheels turn in their favour. The trouble here is that the protagonist is not sympathetically stupid. He is really, really dumb, and not from some sort of lack of schooling or deficiency, just from blithe carelessness, selfishness, failure to spot the very very obvious. He keeps doing very clearly stupid things. This is a problem because, as one stupid move follows another, as his stupidity gets him into scrapes and out of them again and into them again, the reader loses their senses of verisimilitude (surely nobody could be this dumb?) and involvement (if someone is really this dumb, I don't think I care what happens to them), and so doesn’t really care by the time he finally finds some gumption and rescues himself from the worst of the scrapes.
I should, perhaps, note that the name of this stupid protagonist is Nicolas Whistler, which is uncomfortably close to home, which might be why I take so much exception to his rank idiocy. But anyway, even if you can overlook that, there's also some big gaping holes in the plot. Nobody likes pedantic exposition, and nitpicking complaints about tiny gaps in the machinery are tedious, but here there really is a sort of authorial shrug at a couple of pretty crucial moments.
Furthermore, once you're sufficiently irritated by the stupid protagonist and the vacant plot, there's plenty else here you can get irritated at. For example, this is a book in which no bosom goes unnoticed or undescribed---a litany of descriptions irritating in their frequency, their obtrusiveness, and their vague insufficiency (what exactly is a "bomb-like" breast? Are we talking a sort of classical spherical thing with a long fuse, or one of those sleek pointy ones with fins, or perhaps something more like a grenade? The niche campaign group Perverts for Precision demands answers).
All in all, I was not impressed by this one. I'm not sure how much you should trust my judgement—plenty of other people seem to really like it—but I note with mild dismay that I have two more Davidson books to look forward to in this series.