The third section of the work opens with three italicized paragraphs that appear to be from the point-of-view of Grace Poole, the character from Jane Eyre hired to look after the madwoman in the attic. Speaking to another servant by the name of Leah, Grace explains that she was hired by a woman she calls "Mrs. Eff" (Jane Eyre's Mrs. Fairfax) and paid a significant amount of money to look after a "girl who lives in her own darkness." Mrs. Eff instructs Grace that "there must be no gossip" about her charge, a Creole lady who was brought back to England by the master of the house. Grace insists that she does not breathe a word of what is going on, and explains that she chooses to stay on in the house because it provides "a shelter from the world outside which... can be a black and cruel world to a woman." She speculates that this is why all of the female servants remain, but notes that the woman she looks after is not content with her restricted existence. Grace says she fears the fierce look in the girl's eyes.
Antoinette then takes up the narrative responsibilities for the remainder of the novella. She awakens just as Grace Poole is lighting a fire and she watches it burn with a great deal interest. As she stares at the flames she wonders why she has been brought to this attic and why she has been forced to stay there for so long. It is unclear precisely how many weeks or months have passed since her arrival - at one point she says "nights and days and days and nights, hundreds of them" - but she has had definitely time to develop a routine. Each night she pretends to be asleep while Grace Poole drinks and counts out a pile of money. When Grace finally falls asleep, Antoinette sneaks out of bed, removes the key from around her caretaker's neck, and wanders about the darkened house. On one such night she sees a girl in a white dress, probably Jane Eyre, who becomes convinced there is a ghost in the house.
Antoinette's account reveals just how confused and disoriented she has become during her period of confinement. She thinks she beholds her mother in the tapestry in the next room, she describes hearing voices in her head, and she refuses to believe she is in England despite everything she is told. She recalls the ocean voyage to her current destination and remembers asking one of the crew members to help her. The man responded by summoning Rochester, and Antoinette became overwrought and had to be given a sedative. She insists that the ship must have gotten lost on its way to England.
One morning Antoinette awakens aching all over but with no memory of what transpired on the previous day. Grace tells her that a man, her stepbrother Richard Mason, came to see her. Antoinette at first insists that she knows no such person but then recalls writing a letter to Richard in which she begged him to rescue her. She runs around frantically searching for the missive, which she had hidden away, but Grace tells her it is no use: Richard will never be coming back. Antoinette evidently attacked him with a knife after he said he could not "interfere legally" between her and her husband.
Antoinette then races about the room in search of the red dress she brought with her from Jamaica, the dress she was wearing the last time she saw Sandi. She wistfully remembers the way he kissed her when they said 'goodbye' to one another. The sight of the dress lying on the floor makes her think of fire spreading across the room, which in turn reminds her of something that she must do. She says that she will do it "quite soon now."
That night she dreams for the third time that she has stolen the keys from Grace and ventured into the main part of the house. She takes a candle with her and goes to "a large room with red carpet and red curtains" where she lights more candles. In her dream, she becomes anxious when she thinks that someone is going to find her in the room, which then suddenly turns into Aunt Cora's room. Antoinette becomes angry and knocks all of the candles down. They catch the curtains and fire begins to spread. Still dreaming, she goes up to the top story of the house and stands on the battlements where scenes of her past life flash before her eyes: Jamaica, Coulibri, the parrot Coco, Tia, Christophine, and Rochester all flash before her eyes.
She wakes up from the dream and calls out. Grace Poole comes over to see what is the matter but Antoinette pretends to be sound asleep. As soon as she hears Grace snoring again, Antoinette gets up, takes the key, lights a candle, and steps out into the passage. She is going to reenact the dream and burn down the house, thus bringing Wide Sargasso Sea to the same conclusion as Jane Eyre.
Grace Poole's account is important because it is the only part of the narrative from the point-of-view of a relatively unbiased third party. Grace has never met Rochester and knows little or nothing about what happened between him and Antoinette in the Caribbean. It is not clear whether her brief moments as narrator are conversational fragments overheard and filtered through the consciousness of Antoinette; this is certainly possible, as Grace at times seems to be speaking to another servant named Leah. Grace's monologue reiterates several of the work's major themes, most notably the idea that money is the root of evil. "I don't serve the devil for no money," Grace says, but Mrs. Eff insists that Rochester is "gentle, generous, [and] brave," although his stay in the West Indies has altered him almost beyond recognition.
Grace explains that Rochester's father and brother have died and as a result he has inherited all of the family fortune; however, as she points out "he was a wealthy man before that." Ironically his marriage to Antoinette - a union he entered for purely fiscal reasons - has turned out to be unnecessary. Only by depriving Antoinette of her financial independence was Rochester able to assert his own independence from his father and older brother, and now that he no longer needs her money it is far too late. Antoinette has paid for his transgressions, both with her dowry and with her sanity.
The final section of the novella overlaps for the first and only time with the storyline of Jane Eyre, and by this point, Antoinette has effectively become Bertha, the madwoman originally created by Charlotte Brontë. Although her portrait of Antoinette remains sympathetic, in this part of the work Rhys reverts largely to Brontë's characterization. If Antoinette was not mad before, being confined in the attic has certainly rendered her so. She no longer seems to know who or where she is, and Richard Mason even fails to recognize her when he comes to visit. Moreover, Antoinette has become violent and unruly, just as Brontë portrayed her. The conclusion, in which she prepares to burn down the house, comes as no surprise.
Nevertheless, Wide Sargasso Sea stands as a literary masterpiece even apart from its connection to Jane Eyre. In what is perhaps her most significant addition to Brontë's work, Rhys again raises the possibility of an incestuous relationship between Antoinette and her half-brother in the final pages of the novella. In one of her only lucid moments, Antoinette mentions Sandi in a way that could easily be construed as sexual. She explains that he used to come to see her when "that man" (probably Rochester) was away, and that the last time he did so she was wearing a dress of bright red fabric, the color symbolically associated with passion and lust. The two shared a "life and death kiss" before parting, Antoinette says, and this too suggests an element of romance between them.
Like the rest of the work, the depiction of Antoinette in the final pages of the work retains certain autobiographical elements from the life of Jean Rhys. In her letters Rhys writers that she identified strongly with the madwoman she created. Indeed, she certainly knew what it was like to live an isolated existence. At the time of Wide Sargasso Sea's publication in the 1960s, Jean Rhys was living as a recluse in a remote corner of England and drinking herself into a stupor. Because of her increasingly antisocial behavior during this time, she was actually ordered by the court to seek out the care of mental health practitioners. Scholars and biographers have long pondered Rhys's prolonged absence from the literary scene while simultaneously noting that her period of seclusion led to the creation of her greatest work. Critics Sandra M. Gilbert and Susan Gubar have suggested that the figure of the madwoman in the attic that haunts the pages of nineteenth-century literature figures as an emblem of the woman writer. Perhaps this is quite literally true in the case of Jean Rhys; that is, perhaps Rhys, like Antoinette, was seeking a safe haven where she could allow her imagination to run free.