This is the story of a childless couple (Anne and Edmund) who have been married for 10 years living comfortable & self satisfied
lives in a charming house on the river near somewhere near Henley on Thames. They are financially secure, Edmund being the the junior partner in an upmarket London estate agency.
Their lives are disturbed when Cara, Edmund's much married stepmother, telephones him from the South of France asking if he and Anne could look after her 22-year daughter Arabella for a time while she, her mother, on holiday with her latest husband, sorts out one or two marital prospects for her daughter. They accept her but inevitably a brief sexual relationship between Edmund and Arabella in their first week fouls things up.
Hilary Mantel enlarges on this plot (and the minor characters) in a concisely written introduction.
As the title suggest it is Arabella who eventually loses out. Literaturewise it is a beautiful piece of work, but I couldn't warm to it.
I prefer to read about real grief in the quality press.