Eugénie Grandet is set in the town of Saumur. Eugénie's father Felix is a former cooper who has become wealthy through both business ventures and inheritance (inheriting the estates of his mother-in-law, grandfather-in-law and grandmother all in one year). However, he is very miserly, and he, his wife, daughter and their servant Nanon live in a run-down old house which he is too miserly to repair. His banker des Grassins wishes Eugénie to marry his son Adolphe, and his lawyer Cruchot wishes Eugénie to marry his nephew President Cruchot des Bonfons, both parties eyeing the inheritance from Felix. The two families constantly visit the Grandets to get Felix's favour, and Felix in turn plays them off against each other for his own advantage.
On Eugénie's birthday, in 1819, Felix's nephew Charles Grandet arrives from Paris unexpectedly at their home having been sent there by his father Guillaume. Charles does not realise that his father, having gone bankrupt, is planning to take his own life. Guillaume reveals this to his brother Felix in a confidential letter which Charles has carried.
Charles is a spoilt and indolent young man, who is having an affair with an older woman. His father's ruin and suicide are soon published in the newspaper, and his uncle Felix reveals his problems to him. Felix considers Charles to be a burden, and plans to send him off overseas to make his own fortune. However, Eugénie and Charles fall in love with each other, and hope to eventually marry. She gives him some of her own money to help with his trading ventures.
Meanwhile, Felix hatches a plan to profit from his brother's ruin. He announces to Cruchot des Bonfons that he plans to liquidate his brother's business, and so avoid a declaration of bankruptcy, and therefore save the family honour. Cruchot des Bonfons volunteers to go to Paris to make the arrangements provided that Felix pays his expenses. The des Grassins then visit just as they are in the middle of discussions, and the banker des Grassins volunteers to do Felix's bidding for free. So Felix accepts des Grassins' offer instead of Cruchot des Bonfons'. The business is liquidated, and the creditors get 46% of their debts, in exchange for their bank bills. Felix then ignores all demands to pay the rest, whilst selling the bank bills at a profit.
By now Charles has left to travel overseas. He entrusts Eugénie with a small gold plated cabinet which contains pictures of his parents.
Later Felix is angered when he discovers that Eugénie has given her money (all in gold coins) to Charles. This leads to his wife falling ill, and his daughter being confined to her room. Eventually they are reconciled, and Felix reluctantly agrees that Eugénie can marry Charles.
In 1827 Charles returns to France. By now both of Eugénie's parents have died. However Charles is no longer in love with Eugénie. He has become very wealthy through his trading, but he has also become extremely corrupt. He becomes engaged to the daughter of an impoverished aristocratic family, in order to make himself respectable. He writes to Eugénie to announce his marriage plans, and to break off their engagement. He also sends a cheque to pay off the money that she gave him. Eugénie is heartbroken, especially when she discovers that Charles had been back in France for a month when he wrote to her. She sends back the cabinet.
Eugénie then decides to become engaged to Cruchot des Bonfons on two conditions. One is that she remains a virgin after marriage, and the other is that he agrees to go to Paris to act for her to pay off all the debts due Guillaume Grandet's creditors. Cruchot des Bonfons carries out the debt payment in full. This comes just in time for Charles who finds that his future father-in-law objects to letting his daughter marry the son of a bankrupt. When Charles meets Cruchot des Bonfons, he discovers that Eugénie is in fact far wealthier than he is. During his brief stay at Saumur, he had assumed from the state of their home that his relatives were poor.
Cruchot des Bonfons marries Eugénie hopeful of becoming fabulously wealthy. However, he dies young, and at the end of the book Eugénie is a very wealthy widow of thirty-three having now inherited her husband's fortune. At the end of the novel, although by the standards of the time she should be unhappy – childless and widowed – she is instead quite content with her lot. She has learned to live life on her own terms, and has also learned of the hypocrisy and shallowness of the bourgeois and that her best friends will come from the lower classes.

Question: Who is Charles having an affair with?
Answer: An woman older than him.