Wide Sargasso Sea begins in Jamaica after the Emancipation Act of 1833, under which Britain outlawed slavery in all its colonies. The first part of the novella is told from the point-of-view of Antoinette Cosway, a young white girl whose father, a hated former slaveholder, has died and left his wife and children in poverty. The family's estate, Coulibri, is quickly falling into ruin, and Antoinette's mother, Annette, is rapidly sinking into a deep depression. Since her mother frequently rejects her, Antoinette spends most of her time alone or with her black nurse, Christophine, one of the few servants who has not chosen to desert the struggling family.
One day, for the first time in a long time, visitors come to Coulibri. One of these men, an Englishman by the name of Mr. Mason, proposes to Annette after a short courtship. She accepts, and the two wed in spite of the malicious gossip of the servants and local islanders. For a while things seem to be improving for the Cosways: Mr. Mason uses his wealth to restore the crumbling Coulibri plantation, and this in turn seems to improve Annette's mental state. Still, Antoinette's mother repeatedly expresses a desperate wish to leave Jamaica. She is acutely aware of the fact that the freed blacks still harbor immense hatred toward the white aristocracy that enslaved them. Mr. Mason, however, fails to realize how dire the situation has become. One night, a mob sets the house on fire, and the family is forced to flee forever.
Antoinette wakes up several weeks later at the home of her Aunt Cora in Spanish Town. She learns that her brother has died and that her mother has had a mental breakdown. Aunt Cora enrolls Antoinette in a convent school, where she spends several years learning how to be a lady. During this time Antoinette is largely alone; her mother is confined to the home of a care-taking couple, her aunt returns to England, and her stepfather travels frequently and visits rarely. Then, when Antoinette turns seventeen, Mr. Mason comes to the convent and announces that he has friends coming from England for the winter. He implies that one of these men will marry Antoinette.
The second part of the narrative opens after the marriage has taken place. This section of the work is narrated mostly by Antoinette's new husband, a man who remains nameless throughout the text but who is clearly based on the character of Mr. Rochester from Charlotte Brontë's Jane Eyre. As the couple journeys to their honeymoon house, Rochester explains the circumstances that necessitated his hasty wedding. Evidently Rochester arrived in the West Indies and was immediately struck by the fever; as a result he is now questioning his decision to marry a woman he barely knows. Still, he reflects, there was a tremendous incentive for him to do so: his new wife's stepbrother has given him unconditional control of her entire dowry. This money enables Rochester, who is a second son and stands to inherit nothing under the English law of primogeniture, to be financially independent, which is crucial since he has apparently accrued some dishonorable debt.
The wedding party arrives at Granbois, Antoinette's inherited property on another island where she spent much of her youth. Rochester is overwhelmed by the scenery, distrustful of the servants, and generally displeased with the honeymoon house. Antoinette tries to reassure him and help him understand the Caribbean way of life, and for a while this seems to work. Several weeks pass reasonably happily, as the two get to know one another through conversation and finally through consummation of their marriage. Both soon become addicted to sex.
Then one day Rochester receives a letter from a man who calls himself Daniel Cosway and insists that he is Antoinette's illegitimate half-brother. In his letter, Daniel tells Rochester that the Englishman has been tricked into marriage with a madwoman, and encourages Rochester to come and visit him to get the full story. Rochester believes what he reads, and when he returns to the house Antoinette and the servants can sense that his attitude toward her has changed.
The point-of-view then shifts back to Antoinette, who is journeying on horseback to visit the wise old servant Christophine, a woman who is rumored to practice the dark art known as obeah. Antoinette explains that Rochester has become cold and distant, and begs her former nurse to use black magic to make him love her again. Christophine resists, suggesting that Antoinette leave her husband instead. Antoinette refuses, however, noting that under English law all of her money now belongs to Rochester. Christophine, appalled to hear about Antoinette's utter dependence on Rochester, finally agrees to help her.
The perpective then shifts back to Rochester, who receives a second missive from Daniel Cosway and goes to pay the man a visit. Daniel immediately begins to deride the Cosway family, implying, among other things, that Antoinette has had an incestuous relationship with her half-brother Sandi. Daniel attempts to bribe Rochester, saying that he will keep quiet about these matters for a fee. Rochester is disgusted and leaves, but is clearly affected by the encounter.
Back at Granbois, Antoinette confronts her husband about his hatred of her, and he admits that he has been to see Daniel. Antoinette attempts to explain her family history to him but in doing so becomes highly distraught. Rochester suggests that she retire for the night so that they can talk when she is more "reasonable." She agrees but asks him to come to her room. He obliges, and she slips the powder that Christophine gave her into his drink.
Rochester awakens in his wife's bed the next morning and realizes that he was drugged and that the two of them have slept together again. He is sick to his stomach and then furious, and retaliates by seducing the servant girl AmÃ©lie within Antoinette's range of hearing. Antoinette is traumatized by her husband's infidelity and disappears for several days. She returns in a state of drunken dishevelment, and verbally lashes out at Rochester for what he has done to her. She loses control of herself entirely.
Christophine also comes to the house and accuses Rochester of psychologically destroying Antoinette, who has been reduced to throwing objects and trying to bite like an animal. The old servant begs Rochester first to try to love his wife again, and then to go back to England without her. Rochester briefly considers the latter, but changes his mind when Christophine suggests that eventually Antoinette might find marital felicity with someone else. He becomes enraged and orders Christophine to leave the premises, threatening to call the police to report her practice of obeah. She has no choice but to go.
Rochester then begins to make plans to return to Jamaica and consult with the doctors there about his wife's unstable mental condition. Symbolically, he sketches an English-style house with a woman standing in one of the third-floor rooms. They depart several days later, and Rochester insists that Antoinette will never see the island again. She betrays no emotion when they leave, but a young boy native boy cries pitifully because he wants Rochester to take him along. Of course, Rochester refuses - he hates everything belonging to the Caribbean.
The short final portion of the novella starts off with a few paragraphs from the perspective of Grace Poole, the woman hired to guard Antoinette in the attic where she has been imprisoned. Grace reveals that she is being paid well for her services. Ironically, Rochester's father and brother have died and left him everything.
The point-of-view then shifts one last time, back to Antoinette. Her account reveals that she is extremely confused and disoriented: she does not know where she is and has no idea how long she has been there, and furthermore she has only vague, fragmented, and conflated memories of events both recent and long-past. Now, perhaps, she really is a madwoman. At night, she explains, when Grace Poole is asleep, she steals the keys to the attic and sneaks about the house. One night, after a recurring dream of fire, she gets up, takes a candle, and prepares to burn down the house.