A book that starts a bit like an Enid Blyton story of a boy, Roger, and his twin sisters driving with their parents to a holiday in France, but soon becomes something a little strange and quite depressing. The crumbling chateau, situated somewhere near La Rochelle & Niort in the Vendees, is owned and let by a Mssr. Serpe (if the snakes on the cover didn't tip you off). Mssr. Serpe has a daughter (or daughters..? Part of the mystery), Melusine, who lives with him in abject poverty, tending to their herd of goats, while they rent out the remaining grand rooms to guests. There's a mysterious locked tower, a lost road, a secret tunnel, and Serpe seems a right villain.
Like I said, Enid Blyton-esque, until it isn't. This book could warrant a sexual assault-trigger warning. There's also a scary near-drowning and a serious car crash thrown into the mix. And on top of that, some . . . well, it's either magical realism or pure fantasy. I'm leaning towards the latter, as Roger knows what he has seen and experienced can't be real, though it was (there was a disturbing bit with a psychologist towards the end).
Roger's parents are likeable (and competent), and Mssr. Serpe is quite another sort. There's a slimy lawyer in there as well, an accidental death, trespassing, an attempted murder with a rifle at close-range, and then the snakey stuff which I won't spoil. Lots of French dialogue as well, and it's written around well (if you can read it, bon. If you can't, you'll be able to get the gist without it being pedantically translated).
The story concludes in an open-ended manner, and I found it to be gratifying. There have been a few good Puffin Plus Thrillers I've read, and this is another.