Greenmantle is at its core a straight ahead espionage story. Richard Hannae and a few of his other public school colleagues (and a rough-hewn Afrikaaner tracker) are sent into middle Europe in disguise to discover the source of rumours of some kind of prophet who will unite the Moslem allies of Germany and turn the tide of the war. There are several really exciting sequences such when he is on the run in the German countryside or trying to maintain his identity while being heavily scrutinized by an enemy agent. There are also several really weird sections made weirder by the war is glory/just good sport that all men long for propaganda. The reaction of the heroes to the female antagonist is also really twisted and full of bizarre sexual dread. There is a very good piece here on the book that goes into in much better depth that is worth reading if you are interested.
One of the things that I found particularly difficult to swallow was how all these various British public school boys could not only go completely undercover in Arabic cultures, but also end up as their spiritual leaders. It really is the stupidest kind of colonial "privilege". Still, most of Greenmantle is really fun, just keep your critical monocle on as you read it.