The narrative begins with the protagonist, Annabelle Millars, reflecting on her son Cain's fifteenth birthday, a significant milestone marking his entry into adulthood. However, the celebration is marred by the utter destruction of their home, a testament to a wild party. Annabelle finds herself alone as her son has gone out with friends and her husband, Simon Evans, has left for his mistress's home. Her mother-in-law, Berta, has also departed, leaving only a curt instruction about breakfast. As the clock strikes midnight, Annabelle places three letters on the table and leaves the house with a prepared bag, uttering a cold farewell.
Annabelle's marriage, which began at seventeen with Simon, a merchant's heir, was far from happy. Despite her noble background as a baron's daughter and his less affluent status, she married him out of love, overcoming her family's initial objections, though her brother remained opposed. The marriage was conditional, with her parents withdrawing all financial support. Initially, Annabelle struggled with domestic chores, having never performed them before. Simon's mother, Berta, deeply doted on her son and resented Annabelle, viewing her as an incompetent wife who had "stolen" her son. Despite Berta's constant criticism, Annabelle diligently learned household tasks, though she never earned her mother-in-law's approval.
A year into the marriage, Annabelle became pregnant and gave birth to a son, Cain. The joy of motherhood was short-lived as Berta immediately took Cain away, claiming it was the family custom. Simon, siding with his mother, refused to intervene, leaving Annabelle deeply disappointed. She was then burdened with the household chores of her in-laws' home.
Six years into the marriage, a plague swept through the land, claiming the lives of Annabelle's parents and her father-in-law. Simon inherited the merchant guild prematurely, and Annabelle, despite her lack of experience, stepped in to manage the business, proving surprisingly adept. This success, however, strained her relationship with Simon, who became openly displeased with her involvement in his work. Annabelle's creation of a new popular product revitalized the guild, attracting noble clients, but this further widened the rift between them, leading to Simon's affair with a new employee.
Annabelle discovered Simon's infidelity through subtle clues and eventually overheard a conversation between him and his mistress, revealing his contempt for Annabelle, his marriage for financial gain, and his belief that she was a "pushover" who would never divorce him. This revelation shattered Annabelle's love for Simon.
When Cain turned ten, Annabelle wished for him to attend school, an expensive institution for the wealthy. Simon refused to contribute, demanding Annabelle pay the full tuition herself, driven by his resentment of Cain surpassing him. Annabelle sacrificed her own needs to afford the fees. One evening, she overheard Cain, influenced by Berta and Simon, expressing shame about his mother's appearance and lack of wealth, wishing his mother were Simon's mistress, Lara. This deeply wounded Annabelle, confirming her isolation and lack of appreciation within the family. She resolved to leave on Cain's fifteenth birthday, the day after he became an adult, to fulfill her parental responsibility until then.
The morning after Cain's fifteenth birthday, Berta discovers Annabelle's absence and the ransacked house. She finds three letters addressed to her, Simon, and Cain. Annabelle's letter states she is leaving and wishes them happiness. Berta dismisses it as a ploy for attention, confident Annabelle will return.
Cain, returning from a party, is disgusted by the state of the house and blames his mother. He finds a letter from Annabelle stating she is leaving and wishing them happiness, but he quickly dismisses it as insignificant, rushing to school.
Simon, returning from his mistress's house, is furious at the mess and his wife's perceived incompetence. He finds Annabelle's letter, but believes it's a jealous reaction to his affair and that she will return. He is unconcerned about divorce, as it would negatively impact his business. He plans to hire help and make Annabelle pay for it.
Two days later, Annabelle arrives at her brother's estate, where she is warmly welcomed. She apologizes for her past choices, and her brother reassures her. She visits her parents' graves, expressing gratitude and regret. She informs her brother of her plan to travel, starting with the country of Seine to meet a contracted artist. Before leaving, she gives her brother a letter to give to Cain if he ever visits, a final act of maternal responsibility.
Three days after leaving, Simon receives a divorce certificate from the church, confirming his divorce from Annabelle. He is shocked, as he never signed a divorce paper and believed Annabelle loved him. He learns from the church that a divorce paper was signed as a condition of their marriage years ago. Realizing he has been outmaneuvered, Simon decides to conceal the divorce to maintain his business's reputation, planning to tell his mother and son that Annabelle is ill. He finds a final letter from Annabelle, stating she is leaving and wishing them happiness, realizing too late the consequences of his actions.
A week later, Annabelle arrives in Seine. She seeks out the artist she contracted with, Cassius, finding his atelier in a peaceful, natural setting. Upon meeting, Cassius, now a grown man of twenty-four, is surprised and initially flustered by her arrival. Annabelle realizes how much he has grown and they share a heartfelt reunion, acknowledging their mutual desire to see each other.