In 1910 London, England, Kate Croy (Helena Bonham Carter) lives under the strict supervision of her wealthy and controlling Aunt Maude. Aunt Maude, played by Charlotte Rampling, has taken Kate into her home with the primary intent of arranging a wealthy marriage for her, hoping to spare her from the same fate that befell Kate's deceased mother—a woman who suffered greatly due to her union with Kate's reckless and estranged father, Lionel, portrayed by Michael Gambon, who squandered the family fortune on his addiction.
As events unfold, the charming aristocrat Lord Mark (Alex Jennings) begins to pursue Kate with her aunt's blessing. Deep down, however, Kate harbors feelings for a passionate young journalist named Merton Densher (Linus Roache), whom her aunt disapproves of due to his lowly status. Despite the prohibitions, the two have been secretly meeting, but Merton's growing impatience for their future together weighs heavily on both their hearts.
As tensions rise, Aunt Maude confronts Kate about her ongoing relationship with Merton and threatens to withdraw her financial support, prompting Kate to sever ties. A few months later, during a dinner party hosted by Aunt Maude, Kate meets Milly Theale (Alison Elliott), a spirited American heiress traveling through Europe. Girl-turned-friends, Kate finds herself irresistibly drawn to Milly’s vibrant personality and charm. Meanwhile, she and Merton secretly rekindle their romance.
With an opportunity to travel to Venice with Milly and her companion, Susan Stringham (Elizabeth McGovern), Kate sees her chance. However, Lord Mark discreetly informs Kate that Milly is ill and bears the burden of marrying her to secure his inheritance. Recognizing Milly's growing affection for Merton, Kate devises a plan where she invites Merton to Venice, hoping he will woo Milly, with the underlying motive of securing Milly's fortune post-mortem.
As their time in Venice progresses, jealousy begins to creep into Kate’s heart as she witnesses Merton’s deepening bond with Milly. When the two share a night of intimacy, Milly confronts Kate the following day, but Kate cleverly insists Merton is merely a friend. Caught in her web of deceit, Kate realizes that in order for her scheme to manifest, she must leave without a trace.
Alone together in Venice, Milly and Merton's connection flourishes even as her health declines. A surprise encounter with Lord Mark leads Merton to discover that Kate has exposed their secret, leading to a new wave of chaos. Despite the tension, Milly forgives Merton, leaving him wracked with emotional turmoil. Tragedy strikes when Milly passes away, and Merton, alongside Susan, attends her funeral.
As time passes, Merton returns to London, only to find himself confronted by Kate one rainy evening. She questions him about his absence and discovers a letter revealing that Milly bequeathed part of her estate to him. Merton, however, refuses to accept the inheritance, insisting that they should marry without the financial windfall. They share a passionate moment, but Kate’s request for reassurance of his feelings for Milly leads to heartbreak. Dejected by the realization of the consequences of her manipulations, Kate leaves Merton, who ultimately finds himself alone, returning to Venice to grapple with his unfulfilled emotions.
