Set in Lancashire, England, in July 1820, the story follows seventeen-year-old Kate Worthington, a spirited young woman desperate to escape her controlling mother. Kate has dreamed for years of visiting Blackmoore, the ancestral seaside estate in northern England belonging to the family of her closest friends, Henry and Sylvia Delafield. She has never been invited because Mrs. Delafield, Henry and Sylvia's socially ambitious mother, has always intended Henry for Miss Juliet St. Claire and views Kate's family as beneath consideration.
When Mama learns Kate rejected a marriage proposal from Mr. Cooper, an elderly suitor, she furiously forbids the trip to Blackmoore and cancels Kate's planned journey to India with her independent Aunt Charlotte. Henry helps Kate devise a scheme to win approval. Mama agrees but imposes a high-stakes bargain: Kate must receive and reject three marriage proposals at Blackmoore. If she succeeds, she may go to India. If she fails, she must do whatever Mama demands without resistance.
A flashback establishes the depth of Kate's bond with Henry. When Kate learned years earlier that she could not visit Blackmoore, Henry promised to bring her there someday and spent an entire summer building a detailed wooden model of the estate for her. The model becomes a symbol of their connection and Kate's longing for escape.
Kate departs after quiet goodbyes to her seven-year-old brother Oliver, the one family member she feels genuine tenderness for, and her distant father. She arrives to find Mrs. Delafield openly hostile, while Miss St. Claire greets her with patronizing warmth, acting as though she is already the estate's mistress. Sylvia reveals that Henry intends to propose to Miss St. Claire and asks Kate to stay in her room that first evening so she will not distract him. Kate is placed alone in the old, reputedly haunted west wing.
Kate discovers a forgotten music room containing a gilded birdcage with a dark bird that flutters frantically but never sings. She feels an immediate kinship with the caged creature and claims the room as her sanctuary. Her first attempts to secure proposals fail: One man publicly mocks her travel plans, and she unwittingly flirts with the man Sylvia has romantic feelings for. Sylvia reveals that Kate's older sister Eleanor's scandal in London has tainted the Worthington name so thoroughly that no guest would consider a marriage proposal. Sylvia accuses Kate of acting just like Mama, manipulating people for personal gain. Kate retreats to the bird room and realizes that in trying to avoid becoming like her mother, she has adopted her mother's tactics.
Herr Spohr, a German composer visiting Blackmoore, interrupts Kate's frantic playing of Mozart, declaring she is fighting the music rather than expressing herself, and confiscates her sheet music. That night, Henry finds Kate weeping before the birdcage. She confesses her terror of being permanently caged by her gender, her family, and her powerlessness. Henry opens the cage and places the bird in her hands. When she releases it, Kate laughs with sudden, inexplicable joy.
Sitting on the moors before dawn, Kate realizes a loophole: Mama specified three proposals, not three different men. She resolves to ask Henry to propose three times, declining each time. Henry agrees but sets his own terms: one proposal per night, and as payment Kate must share three secrets explaining what changed her from "Kitty" to "Kate" two years earlier.
Henry leads Kate through a secret passageway to the highest tower of a ruined abbey, a private sanctuary he has never shown anyone. Over three nights the bargain unfolds as an increasingly intense exchange. Kate's first secret reveals she witnessed her mother flirting publicly with a militia captain, which sparked her determination never to become like Mama. The second night, Henry asks why Kate fears love. She denounces it as a disease that inevitably turns to loathing. Henry counters that she has only seen love's imitation, traces the curve of her eyebrow, and proposes by pressing his lips to her hand. Kate, trembling, declines.
Crisis arrives when Mama and Maria, Kate's younger sister, appear uninvited. Mama flirts outrageously with the guests, enraging Mrs. Delafield, who threatens to expel them all. On the third night, Kate refuses to reveal her final secret. A rainstorm drives Kate and Henry back through the passage, and in the hallway they nearly kiss before Maria interrupts. Mama, waiting in Kate's room, witnesses the scene and triumphantly declares Henry must marry Kate. Kate protests, shouting she would rather marry Mr. Cooper, and Henry, standing unseen in the doorway, overhears and walks away.
Mama pushes Kate into Henry's grandfather's room the next morning, hoping to pressure the confused old man into changing his will so that Henry could marry a Worthington without losing Blackmoore. Kate physically wrestles her mother from the room when Mama terrifies him. In retaliation, Mama threatens to have Maria entrap Henry and to give Kate to Mr. Cooper.
Kate finds the dark bird dead in its cage. Herr Spohr brings her an original composition and suggests "perhaps it was not its restlessness that killed the bird, but the cage itself" (250). Kate plays the piece, experiences an emotional catharsis, and decides to flee Blackmoore for Aunt Charlotte in London. She enlists her maid Alice to help her escape and writes letters warning Mrs. Delafield about Mama's schemes.
That night, Henry intercepts Kate in the bird room. He proposes genuinely, confessing that the proposal bargain was madness. Kate refuses, weeping, but accidentally says "I do" aloud. Henry kisses her, and she kisses him back before pulling away. She reveals the secret she has guarded for eighteen months: At a ball, she overheard Mrs. Delafield boast to a relative about having Grandfather's will altered so that Henry will lose Blackmoore entirely if he marries anyone Mrs. Delafield disapproves of, particularly a Worthington. This is why Kate declared she would never marry and distanced herself from Henry, even destroying the wooden model of Blackmoore in her anguish. Henry insists he can give up the estate, but Kate refuses, fearing he will grow to resent her. Henry tells her she now knows what it is to be wanted and loved. Kate holds firm, and Henry does not stop her from leaving.
Kate escapes across the moors at night, takes a stagecoach to London, and arrives at Aunt Charlotte's door, sobbing but free. One year later in India, a chance encounter brings Kate a letter from Henry confessing he will love her forever. She begins writing a reply admitting her love has only grown. Then a blackbird's whistle sounds, and Henry appears in the doorway.
He has rejected Miss St. Claire, left Blackmoore in his brother George's care, and taken a position with the East India Company. He has given up his inheritance so Kate can never fear he will resent her. He proposes one final time. Kate falls into his arms and tells him he is her heart's desire. In an epilogue set five years later, Kate, Henry, and their young daughter Olivia sail toward the English coast. Kate points out the great house on the cliff, and Olivia answers: "It's home."