If you don't like a lot of explicit sex, don't read this book. I don't mind myself, but I know that other readers have greater sensitivities than I do.
There is already a good review synopsis given, so I'll try not to repeat that. The book starts in 1815 when Gerard Faulkner, Marquess of Grayson is twenty two. Isabel, Lady Pelham, is a beautiful twenty six year old widow. She is somewhat scandelous as she openly takes lovers, her current lover is Gray's best friend. Isabel's dead husband Pelham was an extremely handsome, charming man, who seduced her straight out of the schoolroom when she was seventeen. Isabel was madly in love with him and thought that her love was reciprocated. However, within six months of their marriage Pelham was off to other beds. Isabel desparately hurt, vows never to love again and when Pelham dies, vows never to marry again. She chooses unmarried lovers and her affairs are all relatively long term, lasting a few years. Unfortunately for her, nearly all of her unmarried lovers fall in love with her and want to marry her, so in the end she ends up hurting them.
Gerard (Gray), a fun loving young rake is in love with his childhood sweetheart, Emily. While off on his Grand Tour of Europe for a year, Gray's mother, a very embittered woman who appears to hate to see him happy, instils doubt in Emily's mind regarding Gray's fidelity. An unhappy Emily marries someone else. When Gray returns he is devestated. However, the two conduct an affair although Gray is not physically faithful.
Gray is attracted to Pel (Isabel), who is a friend. He decides that she would make the ideal wife to spite his mother. He persuades Isabel that if she were already married then her lovers would not want to marry her and despite her fear that she is too old for him, they marry. The bargain is that both continue with their present lives and succession of lovers and neither will interfer. They are good friends and enjoy each others' company. Then Gray receives a letter from his poisonous mother, informing him that Emily and her baby, which is Gray's, have died in childbirth. Overcome with despair and guilt, Gray leaves his London house and Isabel. He spends the next four years 'finding himself'.
Four years later Isabel is shocked to discover that her husband has returned home. Every week for the four years she has written to him, but without reply from him. (I don't quite know how she managed that as she didn't actually know where he was - anyway that's a minor point!).
Gray has changed. Gone is the frivolous carefree young man. The more mature man standing in front of her is serious, harder and determined that he and his wife will be in truth, man and wife. Isabel panics. Gray reminds her so much of her late husband Pelham, whose portrait she keeps in her bedroom to remind her that trusting a man means heartbreak. She believes that handsome men can not be true to their wives because of all the temptation put their way by other women. Although she is now madly attracted to her husband, she fights the attraction by glancing at her late husband's portrait and telling herself that Gray is just like him.
Gray therefore has a battle on his hands to persuade his wife that he has changed, wants only her and will be true to her.
Both characters have past baggage to disperse of. Plus the fact that all through the book, Isabel harps on about the age difference and that she is "old". As she is only four years older than him and men did, and indeed still do, marry older women, e.g. Disralli, I can't see what all the fuss is about. Also included are the minor characters of Gray's awful mother, Isabel's mother and her brother Rhys and his romance and Gray's brother.