Anthony provides Charles with the back-story on Lady Marchmain. She is a strict Catholic and Lord Marchmain converted in order to marry her. When he wanted a divorce, she refused on account of her religion. They are now separated and he lives in Venice with his mistress. He left his wife with everything – both Brideshead and Marchmain House and all the servants who went with them.
Lady Marchmain keeps an eye on her son through her acquaintances at Oxford, particularly Monsignor Bell.
While visiting Lord Marchmain in Venice, Charles gets more info from Cara. She says that Lord Marchmain’s first love was Lady Marchmain, but that now he hates her and everyone who knows her.
Lady Marchmain visits Sebastian and Charles at Oxford. She begins to try and befriend Charles.
We find out that Lady Marchmain is creating a memorial book for her brother Ned, who died in World War I. In doing so she engages the help of Mr. Samgrass, a history don.
After the drunk driving fiasco, Lady Marchmain is surprisingly calm with her son. Still, Sebastian is miserable at having to deal with her, though Charles doesn’t understand why.
Over the Christmas vacation at Brideshead, Lady Marchmain continues her efforts in trying to befriend Charles. She also tries to convert him to Catholicism, which doesn’t help.
From his conversations with her, Charles learns that Lady Marchmain married into money. She used to worry that being rich would get in the way of her religion.
At Easter, Lady Marchmain is not pleased to hear Cordelia report on her son’s drunkenness as the reason for his absence from dinner.
After Sebastian leaves for London, Lady Marchmain takes Charles aside. She is concerned about her son’s depression, and explains that Lord Marchmain used to drink this same way – both men were ashamed of being unhappy.
She asks for Charles’s help in managing Sebastian, and also gives him a copy of the book she’s putting together re: Ned for him to read. Charles realizes she’s trying to manipulate him when he reads the book over.
Lady Marchmain visits Oxford and asks Charles if Sebastian has been drinking. Charles tells her no, but Sebastian is found drunk that very night. Lady Marchmain takes him away from Oxford.
The following Christmas, Lady Marchmain has instructed the servants not to give Sebastian any alcohol. When she finds out that Charles has been sneaking him booze, she guilt trips him hard core.
As Rex later reports, shortly after Charles’s departure she found out that Samgrass was a fake and that Cordelia was also sneaking Sebastian booze. She regretted having yelled at Charles.
Lady Marchmain warns her daughter to stay away from Rex Mottram. When Julia and Rex get engaged, she tries to cover it up while she stalls and plots to drive them apart.
She watches Rex’s conversion with dismay, as he obviously doesn’t take the process seriously, at least spiritually speaking.
Lady Marchmain is devastated when Julia reports that she has been Rex’s mistress for "some time now" and will continue to do so if they don’t get married.
She is further devastated when Julia and Rex are forced to marry under Protestant rites due to his prior marriage.
Lady Marchmain falls ill. She wants to see Sebastian before she dies, but never gets a chance to.