As I understand it, this is one of those archetypical books of an earlier era in which noble, privileged Englishmen ran off to the French Foreign Legion so as to confront adventure and romance. They're all so honest, upright and noble, never noticing for a moment that they're hideously racist and are actively supporting a system of empire in which they think it's the white man's God-given right to plunder the wogs, or something. They euphamize this all by calling it "peaceful penetration". It sounds like a so-called conservative's approach to shrugging off an act of rape. "I didn't rape the woman, your honor, I was merely honoring her by an act of peaceful penetration."
Anyway, this is one of those kinds of books, once considered part of the swash-buckling adventure books all young boys grew up reading (at least in my father's time). It's not bad, really, but the assumed superiority of these people who had everything handed to them on a silver spoon, does get wearying. Of course that attitude is still with us. And also, of course, for the most part, people don't understand that. It's what they grew up with and most of us don't question much. We seem wired to go with the con so long as there's a veneer or silver polish on it.
Whatever, the book is split into roughly three parts. It begins with an officer of the French Foreign Legion coming up to an outpost where all the soldiers are dead, but have been propped up at the parapets so as to seem like the fort is still fully manned. The bugler climbs over the walls to investigate, then disappears. The officer then climbs over the walls, and finds the fort's commander has been bayoneted through the chest. He is clutching a letter, which purports to be a confession to a jewelry theft by one Michael Geste. Michael Geste is the nephew and ward of a Lady Patricia something-or-other, whose estate the French officer has visited a few times. On the way home, he tells his story to a British Officer who has been pining for Lady Patricia for 25 years or so. Naturally, they want to do a full investigation.
Then we cut to a scene of six idle, well coddled young people larking around on Lady Patricia's estate, in between sessions away at school. There are the three Geste brothers, Lady Patricia's daughter, Claudia, the daughter's orphan companion, Isobel, and another hanger on, Augustus, whom everyone hates. At some point, they are admiring a special blue sapphire that Lady Patricia owns. The lights go out, and when they come back on, the sapphire is gone. They do various forms of investigation, but can find neither the sapphire nor the thief. One by one, the Geste brothers disappear, so as to be noble and cast suspicion on themselves and away from everyone else. First goes Michael, the ringleader of the merry band of dependents. Then his twin Digby bolts. Finally, younger brother John, who is also the narrator of much of the story, leaves. They have all run off to join the French Foreign Legion for romance and adventure.
So, the third part of the book is John's telling about the romance and adventure in the Foreign Legion, his finding his brothers, and so forth. There's not much romance and adventure, actually, mostly tedium, tedium in the so-called action and tedium in the telling. Eventually, we get some resolution regarding the jewel theft and realize how truly noble and high minded these young men are, despite their being lackeys of the imperialist system, or something.
As I understand it, Wren wrote a bunch of subsequent off shoots from this book. Perhaps they are indeed filled with swash buckling adventure. This book, while interesting for its significance in helping to understand one's cultural history, seemed to be to be overly filled with people's thinking through their options regarding one method of action or another, and not all that much actual action, romance and adventure.