Charles passes Julia on her way to Brideshead as he and Sebastian are leaving from his first visit to the estate.
Anthony describes Julia as intelligent, beautiful, and a fiend.
Charles officially meets Julia when he’s hurrying to Brideshead under the impression that Sebastian is dying. She informs him that Sebastian merely broke a small bone in his foot. Charles is captivated by her similarities to her brother.
Julia comes to visit Charles and Sebastian at Oxford, bringing Rex Mottram with her. She and Rex invite the boys to a party.
In the aftermath of the Ma Mayfield fiasco, Julia says she was bored at the ball and the men should have taken her with them – after all, she’s always wanted to see Ma Mayfield’s.
When Charles tries to broach the topic of Sebastian’s alcoholism with her, Julia is flippant and uncaring, calling her brother "boring."
Julia is with the others at Brideshead over the Christmas holiday, just after Sebastian has returned from Europe with Mr. Samgrass and Lady Marchmain is trying to stop his drinking. Again she responds to her brother’s alcoholism with casual annoyance. Charles concludes that she cares more about the family’s reputation than his health or happiness.
Julia receives a tortoise from Rex with her initials placed in the shell in diamonds.
Rex later reports that Julia realized Samgrass was lying about Sebastian and called him on it in front of her mother.
After Rex discusses his wishes to marry Julia, Charles explores her character. He claims he is so interested in her because she is so like Sebastian. When he first met her, she was 18 and trying to decide whom to marry. Rex’s darker side appealed to her.
Charles then recounts the way she and Rex met. He was having an affair with Brenda Champion and Julia was staying nearby with her Aunt. After they met, Rex pursued her relentlessly.
One night, after flaking on hanging out with Julia, Rex spends the night with Brenda. Julia finds out. When Rex comes to make amends, they end up engaged. Julia explains that the only way she could justify her anger at Rex for sleeping with Brenda is by being involved with him herself.
Julia and Rex start hooking up until Julia has a moral and religious crisis and cuts him off. So Rex gets it from Brenda Champion instead. Julia goes to her priest to explain why she needs to hook up with Rex premaritally, but he won’t give her permission to do so. So she says screw it and starts up the monkey business with Rex again.
When Rex can’t get permission to marry Julia from Lady Marchmain, he gets it instead from Lord Marchmain.
Julia gets him to convert to Catholicism, and then has to explain to him that, because he’s already been married, they can’t tie the knot in a Catholic ceremony. So they have it done in a Protestant church instead.
Years later, Julia explains to Charles her reasons for marrying Rex: she was already deeply involved with him and wanted to make an honest woman of herself. She adds that Rex isn’t a complete human being.
Julia hears that Charles is in England again and contacts him, reporting that Lady Marchmain is dying. She asks Charles to fetch Sebastian back to say good-bye to his mother.
Aboard his ship back to England from America, Charles and his wife Celia hang out with Julia. While Celia is bed-ridden with seasickness, Charles and Julia catch up.
Charles believes that Julia’s beauty is more complete than ever, in large part due to the store of sadness she possesses.
Julia fills Charles in on her life. She and Rex tried to have a child, but it was born dead. Rex didn’t care because it was a girl. Julia reiterates that Rex is not a whole person, and adds that he doesn’t really love her (nor she him).
The two of them share an obvious ability to communicate beyond words with each other.
Julia explains that she’s lost her religion.
The two of them begin their affair. Before parting ways, they make plans to meet in London.
Julia continues her affair with Charles. Ultimately, they decide to both get divorces and marry each other.
Charles joins Julia at Brideshead.
When her brother Brideshead arrives, Julia learns of his engagement to Beryl. Bridey insults her and her now loose morals, as she’s "living in sin" with Charles while still married to Rex.
Julia, upset, makes her way outside to the fountain. She confesses to Charles that she believes what Rex said to be true.
Later, while they argue about religion, Julia gets suddenly angry at Charles for his attitude and hits him twice before crying and apologizing.
Julia spends time with her sister Cordelia when she comes to visit Brideshead Castle.
Julia has a talk with Charles about his having forgotten Sebastian. She doesn’t like him deeming Sebastian the "forerunner," as she worries she herself is just a forerunner to another love.
Staring at Julia one night, Charles decides that she has regained what first drew him to her that night on the ship, the "store of magical sadness" that seems to say, "Surely I was made for some other purpose than this?"
Julia stays at Brideshead with Charles, Bridey, and Cordelia while Lord Marchmain lives out his last months there. Her father explains that he plans to leave the estate to her and Charles, on account of his disdain for Beryl. Julia doesn’t have a problem with this and doesn’t think Brideshead will either.
Julia fights with Charles regarding her father’s Last Rites. She thinks he should have a priest at his death bed. Julia wins out, and Lord Marchmain even gives the sign of the cross before he dies.
Afterwards, Julia breaks up with Charles. She explains that, in order for God to forgive her, she has to make a sacrifice – and she chooses to sacrifice the happiness she could have had with Charles.
In the epilogue, the lieutenant-colonel reports that the estate still belongs to Julia.
When Charles visits Nanny Hawkins, she reports that Julia is with Cordelia abroad, helping with the war efforts.