The obvious suspect--and the one LeDoux picks--is Derek Esterling. Esterling is a brilliant chemist who has developed a new perfume--which uses ingredients that can also be used as a poison. Esterling is engaged to LeDoux's lovely granddaughter (who will inherit the lucrative perfume business if grandpa dies) and he had quarreled with Charmaron the night before. But Westborough thinks the clues point just a little too conveniently to the chemist. And when another murder takes place the historian is able to unravel the tangled clues and follow the thread directly to a diabolical killer.
When I read this one I felt throughout that I really ought to like it a lot more than I did. That's not to say that I didn't like it--I do. Just not as much as I thought I would. We've got a good, intellectual/academic type for our detective. I love those. The plot has a clever and effective impossible crime. There's a nice bit of misdirection and clueing sleight of hand. The story has a Golden Age country house feel to it--closed set of suspects all gathered together for murder and mayhem. All the pieces are there...and yet it didn't quite knock my socks off. I'm not sure if it's because it seemed to go a bit too long or if it was all about me and the fact that at the time I read it I record that it was an out of sorts kind of week. Regardless--Clason is generally a fine entertainer in the mystery realm and I suggest you give this one a try for yourself.