An old woman is found dead; it appears to be natural causes - a heart attack; except there are a few marks on the corpse which aren't absolutely consonant with that verdict. Brunetti can't ignore them, and is led into an investigation which leads to a dreadful evil behind the death.
If you want thrills and fast action, look elsewhere. To do that, though, would be a shame, because the way Leon tells the story exactly parallels the nature of the crime and the evil: they're hidden things, things society would sooner ignore, things which require careful vision to see.
So the investigation unfolds gently, slowly and not always obviously; yet always inexorably, until we are brought, with Brunetti, to the truth, and the central evil of the book.
It would be a shame if a writer of Donna Leon's class were condemned to write variations on a theme, as so many crime writers do. Each of her books has a different register; she is ready to try new ways of writing and unfolding a plot. And that means, inevitably, that not every one of her fans will like each book. Yet, to me at least, this one is a gem: understated but vital, and never after effect for the sake of effect. So what if Patta et al take a back seat? That's what this story requires, so that's how Leon writes it.
Ignore the nay-sayers and see for yourself...