There are a several interlinked threads in the novel, all of which eventually become revealed bit by bit. The two 'half-brothers', who aren't half-brothers by blood but simply grew up together, are René and Louis. René is over sixty and called 'the old man' ('le vieux'); he has never married and spent all his life in his tumbledown house in the Auvergne, living from poaching and hunting in general, in touch with nature but more in touch with the bottle. Louis, on the other hand, is a comparative sping chicken at fifty-two, has travelled a great deal, loved a great deal, made a great deal of money, and is now in trouble over shady dealings in Africa and returns to the Auvergne to hide himself.
But Louis is more of a good guy than a shady one if compared to Paul Teilhède, who lives in Le Château, which isn't a castle at all but perfectly in keeping with Teilhède himself, who pretends to be a nice guy but is anything but. In fact René is in hospital when Louis calls, the victim of an attempted murder dressed up as an accident that Teilhède's thugs Maurice and Franck inflict on him. Why? Well, René knows a little too much, or rather a lot too much.
And then there's Susan, the English scientist researching foxes and disease, who gets invited to dinner at Teilhède's would-be castle, then invited to a hunt the next day, almost gets raped by Maurice, escapes and falls into the arms of the older Louis, who's of course done it all before but when she falls for him he blows cold so there you are. Maybe.
You see, Susan has also pulled in a visit to the nearby grave of the man her mother wishes she'd married, which of course would have meant Susan would never have been born, but anyway... Brian Allister was a young guy in the RAF during the war, but died in this area of the Auvergne when the plane had to force land. Coincidentally, wouldn't you know, René happened to see this landing, happened to note that Brian had recovered, and led French collaborators (led by Teilhède) back to Brian because if not they might have killed René, an eighteen-year-old Resistance sympathiser. And René has to live for decades with the guilt of shopping Brian and watching Teilhède kill him in cold blood.
But it's many years until Franck, who had little to do with the aggression on René, gets to kill Maurice, gets to kill Teilhède, who used to mock him because he was surely far better off as a hired thug than plucking chickens or breeding maggots?
In the end Louis gets off quite lightly, gets to live in the castle which isn't a castle, and is picking Susan up from Clermont-Ferrand as she's coming to see him for a two weeks, and who knows what will or won't happen? Fortunately, not a sewn-up happy ending, especially as René has died, leaving Louis heartbroken.