John Mortimer can be relied on to turn out out a workmanlike and entertaining piece of fiction and this novel is no exception.
The narrator is an accountant with a passion for amateur dramatics who has a lifelong friendship with the eponymous Dunster, whom he first meets at school and who later keeps turning up at inopportune moments in his life.
Dunster is almost pathologically frank and uncompromising in his pursuit of truth as he sees it. Eventually he seduces and marries the narrator's wife and accuses the narrator's employer, a war hero whom the narrator idolizes, of having committed a war crime in Italy. Much of the book is taken up with the question of this man's guilt or innocence; there is thus a dark undertone to the book which prevents it from being a straightforward comedy of manners, though there is certainly an element of that as well.
I found the denouement fairly easy to predict but that is perhaps how it should be, because part of the irony comes from the fact that the narrator is unable to see the clear implications of what is happening.