Set in Regency-era England, the story follows seventeen-year-old Marianne Daventry, who has spent fourteen unhappy months living with her sharp-tongued grandmother in Bath after her mother's death in a riding accident. Her father left for France to grieve, sending Marianne to Bath and her twin sister, Cecily, to live with a cousin in London. Marianne longs for the country life she left behind.
Two events change her circumstances. Her grandmother disinherits her reckless nephew, Mr. Kellet, and bestows her fortune of roughly forty thousand pounds on Marianne, provided Marianne proves herself an elegant young lady. Almost simultaneously, Cecily invites Marianne to the Wyndham family's country estate, Edenbrooke, in Kent. Cecily mentions her plan to marry the eldest Wyndham brother but never names him. Marianne's maid, Betsy, identifies the eldest son as Charles, so Marianne assumes Cecily intends to marry Sir Charles Wyndham. Lady Caroline Wyndham, the family matriarch and an old friend of the twins' late mother, confirms the invitation. Grandmother gives Marianne a gold locket containing a miniature portrait of her mother, warns her to keep the inheritance secret, and sends her on her way.
The journey turns dangerous when a masked highwayman stops the carriage at night, shoots the hired coachman James, snatches Marianne's locket, and tries to drag her out. Betsy fires a pistol she found hidden in the carriage seat, driving the attacker away. Marianne loads the unconscious James into the carriage and drives the spooked horses through the night to an inn called the Rose and Crown.
At the inn, a tall, handsome gentleman—later revealed to be Philip Wyndham, master of Edenbrooke—mistakes Marianne for a commoner and refuses to help. After she faints on the stairs, he catches her and carries her to the parlor. Over supper, they form an unlikely connection: Marianne pretends to be a dairymaid, and when the man sees through her act, she improvises a ridiculous song about cows that leaves them both laughing. He introduces himself only as Philip, refusing to give his surname. Overnight, he arranges a nurse for James, hires a carriage to Edenbrooke, and pays all the bills without her permission.
At Edenbrooke, Lady Caroline welcomes Marianne warmly, though Cecily has not yet arrived. Exploring the grounds, Marianne twirls by the river in a burst of happiness and falls in. Philip rides up, witnesses her tumble, and she realizes he lives at the estate. She confronts him for concealing his identity, but he deflects with teasing. At dinner, he volunteers Marianne to sing, knowing she fears performing. Lady Caroline scolds him, and Marianne steps in to defuse the situation, establishing an unspoken truce.
Over the following days, they develop a close friendship. Philip offers her a spirited filly named Meg, and they ride together every morning. He shows her the library, his sanctuary, and they spend a rainy afternoon sharing confidences about their lost parents. They make mutual promises: she vows not to love him for his possessions, and he vows not to love her for her misfortunes. Marianne, still believing Philip is a younger brother, thinks of him as a future brother-in-law once Cecily marries Sir Charles.
This understanding shatters when Miss Grace Fairhurst, a neighbor's daughter, reveals that Sir Charles died five years ago. Philip is the eldest brother, a baronet (a hereditary title-holder addressed as "Sir"), and master of Edenbrooke. Cecily's target has been Philip all along. Marianne, who has always avoided competing with her twin, resolves to distance herself before Cecily arrives. She paints a watercolor of the estate with a lone figure gazing at it with longing.
Philip notices her withdrawal and confronts her in the stable, asking if she cares for him. She insists he is only a friend. He writes her a passionate love letter as a "lesson in romance," ostensibly for a matchmaking scheme, but its intensity shakes Marianne. Meanwhile, Mr. Thomas Beaufort, the nephew of one of Marianne's Bath admirers, arrives at Edenbrooke and courts Marianne, provoking Philip's visible displeasure.
Cecily arrives with Louisa and William, Philip's younger siblings, and Rachel, William's wife. Cecily immediately monopolizes Philip and privately tells Marianne she is certain he will propose. Mr. Kellet reappears in the area and flirts with Cecily, who encourages him despite her professed devotion to Philip.
Marianne overhears Philip telling William he wants to be rid of his responsibility toward her and is devastated, concluding his friendship was pretense. At an assembly ball, she tries to shut Philip out, but during a dance, his intensity overwhelms her. She stops mid-step as the truth becomes undeniable: She is in love with Philip. The conviction that he does not return her feelings sends her fleeing in tears.
As Philip and William depart on a trip, Betsy inadvertently gives Philip a raw letter Marianne had written to her father, pleading to know why he abandoned her. While they are away, Beaufort proposes to Marianne, but she refuses. Rachel reads Marianne part of a letter from William describing Philip falling in love with "a real beauty, with nice legs and beautiful lines." Marianne assumes he means a woman, but Rachel clarifies it is a horse at the Newmarket races. Rachel also reveals that Philip gave Meg, a racehorse meant for competition, to Marianne instead, and explains that Philip, as Marianne's host and self-appointed protector, felt honor-bound not to declare his feelings while she depended on his hospitality. Marianne reconsiders everything, realizing Philip may have wanted to shed his role as guardian so he could court her freely.
William returns without Philip, revealing that Philip left the races early after William told him to "go win her."
Beaufort reappears and kidnaps Marianne from the orchard, revealing himself as the highwayman. He confesses that his uncle overheard Grandmother announce the inheritance, that he staged the robbery and shot James, and that he intends to force a marriage to claim her fortune. At an inn, Marianne pretends to cooperate, secretly writes a love letter to Philip and hides it in her boot, then seizes a penknife and Beaufort's pistol. He overpowers her, but the door bursts open. Her father scoops her up and carries her to safety.
Her father explains that Philip traveled to France to deliver Marianne's letter and personally persuade him to return. Philip fights Beaufort with a sword and, at Marianne's request, slashes Beaufort's face as a warning before ensuring he leaves the country. On the carriage ride home, Philip tells Marianne he does not love her for her money, echoing their earlier vows with a telling shift in wording.
The love letter Marianne wrote at the inn goes missing from her writing desk. Cecily finds it, recognizes that Philip loves Marianne, and graciously orchestrates a plan to bring them together in the library. In the darkened room, Philip asks if she meant what she wrote. She whispers yes. He crosses the room, backs her against a bookcase, and kisses her, confessing she stole his heart the night they met and that his honor as her protector kept him from declaring himself until her father's return freed him to court her properly.
The novel closes with the household celebrating the engagement. Marianne's father plans to return to Surrey, just one county away from Edenbrooke. A letter from Grandmother reveals she arranged the visit so Philip could protect Marianne and will leave the fortune regardless of elegance. Philip returns the locket recovered from Beaufort. In the orchard, Marianne proposes a trade: her painting of Edenbrooke for his heart. He agrees. Marianne twirls among the apple trees, tangles her hair in a branch, and Philip frees her before they kiss and walk hand in hand back to the house.