Although he dreams of being an artist, middle-class Charles Ryder finds himself studying history at the illustrious University of Oxford. It is here that he forms a captivating friendship with the extravagant and wealthy Lord Sebastian Flyte. However, their bond faces challenges, especially from Sebastian's mother, Lady Marchmain, who disapproves of her son’s reckless lifestyle, particularly his excessive drinking habits. Upon visiting Sebastian's family home, Brideshead, Charles is awestruck by its opulence and is drawn to its unique inhabitants, including the devout Lady Marchmain and her other children: Bridey, Julia, and Cordelia.
As Sebastian and his sister Julia prepare for a trip to Venice, where their father, Lord Marchmain, awaits with his mistress, Cara, Lady Marchmain encourages Charles to accompany them. She hopes that Charles could provide a stabilizing influence over Sebastian. Amidst this backdrop of burgeoning feelings, Charles finds himself inexplicably attracted to Julia, leading to an impulsive kiss in a dimly lit alley, oblivious to Sebastian spying from across the canal. This act of affection incites jealousy in Sebastian, who then seeks to sever ties with Charles. Upon returning to Britain, Lady Marchmain makes it apparent that Charles's non-Catholic beliefs and self-proclaimed atheism prohibit any potential union with Julia.
Amid growing concerns over Sebastian's deteriorating alcoholism, Lady Marchmain cuts off his financial support. During a visit to Brideshead, when Charles clandestinely offers Sebastian some money for alcohol, it becomes a catalyst for an explosive moment at a family party, which culminates in Sebastian's dramatic exit. In the aftermath, Lady Marchmain chastises Charles and forbids him from visiting Brideshead again, leading Sebastian to escape to Morocco in search of solace.
Fast forward four years—Lady Marchmain is gravely ill and implores Charles to locate her son. Charles journeys to Morocco, only to find Sebastian hospitalized and unwilling to return home. In the ensuing time, both Julia and Charles marry others, yet their lives intersect once more aboard an ocean liner returning from New York. Realizing their undeniable love persists, they make plans to leave their respective spouses in pursuit of happiness together.
Back at Brideshead, Charles intends to confront Rex, Julia's husband, about stepping aside for their relationship. However, Rex's reluctance is compounded when he reveals he converted to Catholicism for Julia's sake, further complicating matters. Julia, overhearing their conversation, feels objectified and hurt, leading to an emotional reckoning.
Just as they prepare to depart Brideshead, the arrival of Lord Marchmain, gravely ill and accompanied by Cara, pulls them back. As he reconciles with his Catholic faith on his deathbed, this profound moment impacts Julia deeply, prompting her to reconsider her love for Charles versus her faith, and they ultimately part ways in sorrow.
Years later, amidst the turmoil of the Second World War, a disenchanted Charles, now an army captain, returns to Brideshead, now repurposed as a military base. Through a chance encounter, he learns that Julia is serving overseas and that her brother Bridey has perished. Alone and reflective, Charles finds solace in the family chapel, where he contemplates the flickering flame of a candle, ultimately choosing to leave the light undisturbed, symbolizing unresolved feelings and the enduring complexity of love and faith.
