Let's not beat about the bush here. This is vintage Wheatley. That means that anybody with half a mind to can pick huge holes in it. It's cornier than a field in Kansas. The characters are all cut from cardboard. There are plot holes you can fly a Miles Hawk through (I'm still wondering what the hell happened to Sabine's mother, since Lord Gavin seems to have got away at the end and he would have been hell-bent on murderous revenge). There are improbable situations. Serious evil is afoot. “For God's sake, man! Get with the plot! This isn't about a few frilly French knickers! It's an evil Communist plot to destroy England!” So, or something like it, exclaims Gregory Sallust to his plodding policeman sidekick. You see, Wheatley desired a particularly evil, unscrupulous, unprincipled villain for this story, and not content to make him simply sinister and deformed, he made him a Communist as well. Because there's nothing more evil, unscrupulous and unprincipled than a Communist, of course. Curiously, this particular Communist does seem to enjoy the finer things in capitalist life, such as casinos, wealth, country estates, private aeroplanes and general all-round fine living. But wait - he's unprincipled. I do wonder why he had to import his saboteurs and agitators? Surely he could have found plenty of indigenous troublemakers without all the hassle of sneaking in foreigners with strange and conspicuous foreign accents to disperse among the disaffected English proletariat? Perhaps Commies are one of the few things foreign countries do better than England. I'm sure Lord Gavin knew what he was doing.
Wheatley has a curious worldview by our modern standards. Exhibit A: the beautiful French girl, having been saved from the clammy clutches of the despicable Lord Gavin, must now be saved from rotting in an English gaol. At any cost. A well-bred specimen like that simply cannot be thrown in the can with all those slags and harlots. It's unthinkable! Have you EVER smelled a women's prison? Just look this way please, Superintendent, while Sallust commits a felony and spirits the lady away. There's a good man. You just come to dinner - I'll have the Home Secretary over, you know. Other bigwigs too. Nudge, nudge...
One assumes Sallust eventually makes it back to England without being charged and imprisoned, because he has other books to appear in. And I'm pleased about that. Because, despite all its glaring faults and curious anachronisms, I had a grand time getting through this ripping yarn.