Set in early 19th-century England, the novel follows Cassandra Austen, the elder sister of celebrated novelist Jane Austen, as she undertakes a final mission to protect her sister's legacy. The story alternates between 1840, when the elderly Cassandra visits the Kintbury vicarage in Berkshire to find and destroy private family letters, and flashbacks triggered by those letters that trace the Austen sisters' intertwined lives from the 1790s through Jane's death in 1817.
A brief prologue establishes Cassandra's story. In 1795, at Steventon Rectory in Hampshire, Tom Fowle, a young curate, or junior clergyman, proposes to Cassandra. She accepts, but their engagement must be long, as Tom earns too little to marry.
In March 1840, Cassandra, now in her late sixties, arrives uninvited at the Kintbury vicarage. The household's patriarch, the Reverend Fulwar Craven Fowle, Tom's elder brother, has recently died, and his unmarried daughter Isabella Fowle is preparing to vacate the house for the incoming vicar. Cassandra carries a small black valise she refuses to let anyone touch and has deliberately timed her arrival to precede her letter of notice, because she has a secret purpose: to locate and destroy any intimate letters she and Jane once sent to the vicarage before they fall into the hands of her sister-in-law Mary Austen, who plans to comb through the family papers for her son James-Edward's potential book about the Austens.
Cassandra feigns frailty to engineer time alone and searches the rooms methodically, though Isabella's maid Dinah watches her with growing suspicion. In the bedroom of Isabella's late mother, Eliza Fowle, a close friend of both Austen sisters, Cassandra discovers an oak settle, a heavy wooden bench with a storage compartment, containing bundles of letters including correspondence from Jane. She hides these under her mattress and reads them at night.
The letters carry Cassandra back through decades. In flashback, Tom announces that Lord Craven, a wealthy patron, has offered him a chaplaincy on a military expedition to the West Indies, promising good pay and an eventual living, a paid church post with a house and steady income. Cassandra fears both the danger and the distance. Tom survives a hurricane and returns for Christmas. Visiting the Fowle household for the first time, Cassandra finds it somber and quails at the prospect of isolation from Jane. On Tom's last afternoon, they visit the church, where Tom reveals he has made his will and asks her not to feel beholden to his memory. Instead, Cassandra vows before God never to marry any man but Tom. After Tom departs again, Eliza goes into premature labor, and Cassandra helps deliver and nurse the baby. Tom dies during the expedition.
Back in 1840, Cassandra is haunted by guilt over her earlier doubts about marrying Tom. Mary Austen descends on the vicarage with her daughter Caroline Austen, bringing grievances and criticism. Among the letters, Cassandra discovers one from Mary dated the day Tom's death was reported, depicting Cassandra collapsing in hysterics, a version that contradicts her own memory of stoic fortitude. She rips Mary's letter to shreds, recognizing how impossible it is to control a family's narrative. Caroline recounts a story Cassandra once told about a mysterious gentleman Jane met at the seaside, and Cassandra warns her never to repeat it, fearing scrutiny of Jane's private life. That night, Cassandra falls gravely ill with fever. Isabella nurses her with surprising skill, saving her life. When Cassandra recovers, the letters she hid under her mattress have vanished.
Returning to the settle, Cassandra cannot find her stolen letters but discovers a second, previously unknown cache of Jane's letters to Eliza, revealing a deeper intimacy between the two women than she had realized. These letters trace the sisters' difficult years: their forced departure from Steventon, the move to Bath, and Jane's anxiety about their uncertain future as dependent, single women.
The most startling flashbacks concern Mr. Henry Hobday. In 1801, at the seaside resort of Sidmouth, Cassandra meets Hobday in a bookshop and feels a powerful, immediate attraction. His mother invites the Austens to call. Cassandra tries to arrange a match between Hobday and Jane, but Jane insists he is plainly in love with Cassandra. An undated note from Jane to Eliza declares with alarming indiscretion that Cassandra is deeply in love. The following summer in Dawlish, Jane and their sailor brother Charles Austen contrive to leave Cassandra and Hobday alone, and he proposes. Cassandra refuses, citing her vow to Tom. Jane is devastated.
Shortly after, while staying at Manydown with friends, Jane accepts a proposal from Harris Bigg-Wither, heir to the estate, not out of love but to secure a home and free Cassandra to marry Hobday. After a sleepless night, Jane retracts her acceptance, and the sisters flee in disgrace. Cassandra writes Hobday a final letter ending all correspondence.
Further flashbacks trace Jane's darkest period. After Tom's death, Cassandra inherits 1,000 pounds, enough for emergencies but not independence. Visiting her brother Edward Austen and his wife Elizabeth in Kent, she is excluded from family dinners and relegated to nursery duties, an experience that crystallizes her resolve to make herself indispensable as the single sister and spinster aunt. After their father's sudden death in 1805, Jane sinks into severe depression, unable to rise from bed for weeks. Cassandra manages the family's finances while Jane struggles with upheaval and lack of privacy for writing. Cassandra identifies 10 letters documenting Jane's depressive episodes and resolves to destroy them, to protect the image of Jane's serene temperament. A turning point comes when Elizabeth dies suddenly after the birth of her 11th child. Cassandra persuades Edward to offer the women a cottage on his Chawton estate, a plan his late wife had vetoed. In 1809, Cassandra, Jane, their mother, and their close friend and household companion Martha Lloyd settle at Chawton, where Jane publishes
Sense and Sensibility and
Pride and Prejudice to acclaim.
Cassandra also pursues a parallel mission in 1840: settling Isabella's future. She persuades Isabella's sisters to share a house, believing the arrangement ideal for three single women. The novel's climactic recognition comes when Dinah hurls herself down the staircase, imitating a dramatic fall from Jane's novel
Persuasion, which Cassandra has been reading aloud. Cassandra sends for the village surgeon, Mr. Lidderdale. As Isabella and the doctor tend to Dinah, Cassandra observes their deep intimacy and realizes she has misread the situation entirely. Isabella is not a lonely spinster but a woman in love with Lidderdale, whose courtship her domineering father forbade because the doctor was not born a gentleman. Cassandra's well-meaning plan was reinforcing the very obstruction the family had imposed. Recognizing she has imposed the lessons of her own life onto another's, she reverses her advice, suggesting Isabella take only temporary lodgings to preserve her freedom.
The final flashback traces Jane's last illness and death on July 18, 1817, with her head in Cassandra's lap. Cassandra performs the last services alone, reflecting that Jane was the sun of her life.
On her last morning at Kintbury, Mary Austen arrives and demands all correspondence from Eliza's room, confirming Cassandra's fears and validating her mission. Departing, Cassandra stops her coach to tell Mr. Lidderdale that Isabella would welcome his call; he smiles and heads to the vicarage at once. In the coach, Cassandra reads Jane's final letter, written from Winchester 10 days before her death, in which Jane calls Cassandra the dearest sister and nurse: "For how could I? What sort of life would it be, if I did not have her by my side?" (272). Cassandra kisses the letter and watches through her tears as the countryside carries her home to Chawton, where she plans to burn the dangerous letters and live out her remaining days in peace.