This is the next of Marsh’s Inspector Alleyn mysteries, and I’m afraid
it’s aged very badly.

Alleyn, his wife, and their little boy Ricky (his first
appearance, as it happens) take a vacation to the south of France.
Alleyn’s mixing business with pleasure; while Troy and Ricky are having
fun, he’s going to be helping the Sureté bust up a drug ring. Tied in
with the drug ring, possibly, are the denizens of the Chateau of the
Silver Goat, the owner of which is the leader of what we’d now call a
New Age cult. It’s a scam, of course, at least mostly, but the cult
leader uses the drugs to keep control over his small flock.

And this is where it gets dated. The two drugs mentioned in the book are
heroin and marijuana, tellingly spelled “marihuana”. Heroin is no joke,
even now, but for the rest this spills over into Reefer Madness
territory. The pinnacle comes during an occult ritual which Alleyn has
infiltrated; there are six other participants. Each attendee is given a
“reefer” to smoke; through a little sleight of hand, Alleyn substitutes
one of his own cigarettes.

Now, really. I’ve never smoked either tobacco or marijuana myself (I was
always a goody-two-shoes) but I know what they both smell like, and if
Alleyn had lit up a normal cigarette in this situation, you can’t tell me
that the other six wouldn’t have noticed the difference.

The book does have some fun moments, including one delightful scene where
Alleyn comes over all Cary Grant and loses his temper with a slimy French
executive, but the first half of the book is a bit of a slog.