It's hard to explain this book, and I don't think it would make any difference anyway. Certainly the plot is the reason for a lot of the book's charm, and it has loads, but it's not because of what happens. I can't really tell you much about Fantomas either, at least no more than the back of the book would tell you. Fantomas is an arch villain and master of disguise, he likes money and enjoys setting elaborate traps the kill in fairly gruesome ways, but really I think he just enjoys spreading terror.. Uh, that's it for Fantomas, I mean he doesn't even appear in the book until at least half way, they really go to town with the whole disguise thing, we don't even get to know who he's pretending to be and it's not exactly obvious, well, it might be if it wasn't for the fact that it could 'obviously' be a few people. Apparently later in the series it's entirely possible that Juve, the smart, but not smart enough, police detective trying to bring him to justice, is Fantomas.
If you're getting the impression that the authors were just making it up as they went along you'd be amazed how right you are. To get the Fantomas stories out regular and often enough Allain and Souvestre worked out a basic plot and then went off and wrote a alternate chapters with very little contact. This is probably the reason for the strange dreamlike atmosphere that makes Fantomas so enjoyable not just to the general public loved the stories but to the surrealists of the time.
It's easy to see why the Fantomas series became such a cult hit, I loved it, I just couldn't honestly tell you why!