The novella opens at a theatre. A beautiful, distinguished, witty lady, who remains unnamed in the text (but here called Fantomina, for ease), is attending the Playhouse. Rather than watching the play, she is observing the behavior of the aristocratic males also attending the theatre, watching how they react to the prostitutes in the ‘Pit’ below. The protagonist muses as to how some men could have such high class and manners, yet act in such a depraved manner. As an almost social experiment to see how she would be received, the protagonist—who is of genteel birth—decides to dress and act as a prostitute at the theatre. She can get away with acting in this manner as there is no authority figure in the town to whom she is accountable. She watches these ladies with curiosity in order to learn to emulate their behavior herself.
The next night, she enters the gallery box dressed as a prostitute. It is not long before she is surrounded by suitors of all ages and means, vying to pay any price for her attentions. Ironically, one man compliments her by comparing her to her own self, as the Lady. She states she is naturally vain, and is lapping up the attention. She spots Beauplaisir (a name literally meaning ‘fair pleasure’) across the room. She had noticed him before, but her virtuous status as a Lady meant she could make no advances on him. Beauplaisir approaches her in the usual manner, with polite phrases. She answers with wit, and he is charmed to find a woman with the sort of character that is not usually found among prostitutes. Beauplaisir insists that she must satisfy his needs at a nearby apartment, or at his own home. Fantomina is taken aback as to what her ‘Frolick’ has lead to: a proposition for actual intercourse. She makes an excuse that she is already committed to another man for that night, and leaves, promising to be in the same theatre the next night to offer her services.
Fantomina returns the next night, having booked an apartment to invite Beauplaisir back to if the need arises, in order to protect her reputation. She takes Beauplaisir back to the apartment, and orders servants to serve them dinner. This prompts Beauplaisir to think of Fantomina as a high-end mistress. The pair engage in amorous conversation over dinner, and Beauplaisir is eager to claim his sexual satisfaction. Fantomina is confused and scared, yet has gone too far to put him off another night or refuse him altogether. She is on the brink of telling Beauplaisir her real name and rank, when thoughts of the liberty she is currently taking advantage of stop her. Beauplaisir becomes so wild with desire that Fantomina begins to think that even if he were to learn of her genteel rank, it could not stop him. They have intercourse: Beauplaisir is ‘rapturous’ and Fantomina is ‘undone’, having lost her virginity and honor. Beauplaisir offers Fantomina all his estate to compensate for her services, yet she states that this cannot compensate for her loss of honor. Her worrying is evident, and Beauplaisir asks who she is. She cannot tell the truth, so tells her suitor that she is the daughter of a Country Gentleman, come to town to buy Cloth, and that her name is Fantomina. He leaves the next morning, and she promises to receive him at 3pm that day.
She bribes the Housekeeper to pretend that Fantomina constantly lodges there and is from the country. If Beauplaisir calls when she is at her true home, the Housekeeper is to lie, and say that she is currently out. Fantomina then returns home, and tells the aunt she is lodging with that she was out all night with a Gentleman and his Lady on a barge that couldn’t get back in time. When moving through society, Fantomina uses her wit to foresee and prevent any situations that would mean her loss of reputation; however, she is helpless to the loss of her virtue. She meets with Beauplaisir at her rented apartment three or four times a week, but makes sure to never miss social engagements that she regularly attends as a Lady. When dressed as a prostitute, her resemblance to her Lady self keeps Beauplaisir interested. He does muse, at times, about whether the two could be the same person. However, the implausibility of finding a Lady in the company of prostitutes prevents his thoughts from moving forward.
However, according to the predictable temperament of men, Beauplaisir grows bored of Fantomina’s charms. When the opportunity to move among the social spheres in Bath comes up, he makes excuses to go without her. By this point, Fantomina claims she is in love with Beauplaisir, and would do anything to engage in intercourse with him again. Fantomina makes excuses to her Aunt to go to Bath, and sets out with two servants. She finds reason to quarrel with these servants on the road, and dismisses them, leaving herself to do as she pleases. She disguises herself as Celia, in a maid’s uniform, a broad country accent and a generally unrefined demeanor. She then inquires at the house where Beauplaisir is staying at, asking if there is any work for her. The disguised Fantomina takes up the post as a Maid in the house, cleaning the gentleman’s rooms and waiting on them. Aside from a bed-ridden gentleman with rheumatism, Beauplaisir is the only gentleman lodger in the house. Despite the disguise, Fantomina is still exceptionally pretty. As she is cleaning Beauplaisir’s chambers, he notices, and encourages her to sit on his lap, asking her questions such as whether she had ever been in love. Before too long, he cannot control himself and he advances on to Fantomina’s ‘half-reluctant, half-yielding’ body. He pays her a large sum of gold, and bids her be there for him at bedtime. Beauplaisir stays in Bath for a month, and, as with Fantomina, grows weary of Celia’s affections. She senses this, and quits her job to plot another disguise.
Her new disguise is a widow’s dress, with hair tightly done up (she wore it loose as both Fantomina and Celia); she calls herself Widow Bloomer. She knew that Beauplaisir had come by himself in a chariot to Bath, and she is counting on him leaving in the same manner. Disguised as the widow, she places herself at an Inn, and then approaches his carriage when she sees him coming. She makes a tender speech about how she is the unhappiest of all women, having lost her most dear husband. She claims she will lose her savings if she does not get to London in time, and implores him to take her there. Beauplaisir is irritated at the fact that he may have to sit in a carriage with a woman who can talk of nothing but her late husband, but consents. He discovers that she will happily talk of the mutual passion a man and wife have, and describes it in great detail. Beauplaisir offers to relieve her passionate tensions, and the Widow is grateful.
A small digression takes place from outside the action of the story. The narrator explains why Beauplaisir is still being taken for a fool: Fantomina is an expert at disguising her form, her character, and her voice, therefore anyone would be fooled. Beauplaisir muses that he has seen the Widow’s face before, but because he has been told she is from Bristol, he assumes he has never met her before.
The plot continues, and Widow Bloomer and Beauplaisir enjoy each other’s desires in the journey back from Bath. They promise to see each other again, and then part ways. Fantomina writes two letters to Beauplaisir, one as Widow Bloomer and one as Fantomina. He replies to Mrs Bloomer with words of kindness and desire, vowing to see her soon. To Fantomina, he is affectionate yet more reserved. He claims he cannot see her the next day due to a business meeting. Fantomina is ecstatic that her efforts to fool Beauplaisir have worked, but is enraged that he has betrayed Fantomina. She receives him the next day as Widow Bloomer, and Beauplaisir is very enthusiastic. The day after, she receives him as Fantomina and his lack of interest is evident. Fantomina acknowledges, almost as if it is common knowledge, that Widow Bloomer is a more recent acquaintance, and therefore holds more interest. Fantomina believes she loves Beauplaisir to the point of 'excess’, and his visits are now a ‘penance’ rather than a ‘pleasure’.
Realizing that Beauplaisir is growing tired of all her current disguises, Fantomina plots another plan. She goes to the park and employs two gentleman to act as her House staff. She then rents a large house for a week, paying upfront so as not to arouse suspicion. She then sends a letter in a foreign hand to Beauplaisir, posing as ‘Incognita’. In the letter, she compliments Beauplaisir’s charms and invites him round to her new house. She also comments that she must keep her face concealed from her visitor. Beauplaisir replies, stating that he struggles to accept compliments from a woman based wholly on wit, and not on her appearance also. Yet, he agrees to come. Upon receiving his reply, Fantomina laughs that Beauplaisir is so inquisitive, seeing as she has so easily fooled him in the past. She laments for all women that they must deal with this behavior in men.
Fantomina dresses in a spectacular ball gown, but hides her face. Beauplaisir is utterly charmed by her in the evening. His impatience to see her face is calmed by his self-confidence that he will eventually see it once intercourse starts. The amorous man declares his undying passion for Incognita and suggests they go to bed together. He is certain that she will not lie in her mask, and that the light of the morning will reveal her identity. They share a night of passion, and he eagerly awaits the morning. When it does arrive, he is confused as to why it is still dark. Fantomina has arranged blackout blinds to be placed on the windows so he does not see her, and she flees the room before he can protest.
Fantomina continues to maintain her original house, and receives Beauplaisir as both Widow Bloomer and Fantomina. However, it is now extremely evident that he has grown tired of both. She is busy planning a way to drop these two characters and continue the game when her Mother unexpectedly arrives. She has heard nothing of Fantomina’s pursuits. Even so, Fantomina is now restricted in her social freedom. Yet this is the least of her worries: it is revealed she is pregnant with Beauplaisir’s child. Her plan is to eat little and wear huge-hooped petticoats to cover the size of her stomach, until she can be sent to the country to have the baby in secret. She attends a ball as a goodbye from society, and it is here that she goes into labor. Her Mother demands to know the Father of her child. Fantomina tells her Beauplaisir’s name. He is fetched, and does not recognize the Lady as any woman he has laid with. He begs her to admit that she is framing him, and that it is not his child.
Fantomina’s mother eventually tells Beauplaisir that this was a ploy to get him to marry her and save her honor, that it is her own fault, and that he owes nothing to her or the child. He offers to make sure the child is in safe hands if it is discharged to him, but this offer is rejected. Beauplaisir is extremely confused, and continues to visit to inquire of Fantomina’s health until he is asked to stop. Once Fantomina is strong enough, her mother sends her to a French monastery.