In the early 1840s, in Wilkes County, North Carolina, the narrator, Sarah “Sallie” Yates, and her sister, Adelaide “Addie” Yates, travel with their father, David, to the wedding of Fanny, an acquaintance, and Charles Harris. The guests are far more interested in the expected presence of Chang and Eng Bunker, the famous conjoined twins from Siam, who recently moved to the area and are friends with Harris, their manager. The sisters have enthusiastically followed the twins’ lives through newspapers, learning about their physical connection, separate personalities, and the legal and moral dilemmas that their existence provokes.

Sallie contrasts her plain, bookish nature and disinterest in marriage with Addie’s beauty and ambition. Both sisters are of marrying age, but Sallie finds the local suitors dull. Despite their family being wealthy plantation owners, their prospects have been worsened recently by a past family scandal referred to only as “the incident.” As they ride to the wedding, Sallie recalls sensational stories about the twins’ romantic entanglements, and she and Addie wonder about the practicalities of courtship and intimacy for two men physically bound together.

At a wedding reception, Sallie and Addie observe the famous Chang and Eng for the first time. Sallie finds them handsome and refined, moving with a surprising, unified grace. The groom formally introduces the sisters to the twins. The outgoing Chang immediately focuses his attention on Addie and flirts with her openly, which she enjoys, causing Sallie to feel overshadowed. Eng is quieter and more reserved.

During their conversation, the twins explain their unique marital predicament: A surgical separation would likely be fatal, but if only one of them were to marry, the state would consider it bigamy. Chang notes that he and his brother are often attracted to the same type of woman. The sisters’ protective father abruptly ends the interaction and tells them that it’s time to go home. As they leave, Chang makes a parting comment to Addie, suggesting that he hopes to be a suitor for her.

Sallie and Addie attend a housewarming quilting party at Chang and Eng’s newly built estate. They learn that the brothers have become naturalized US citizens, adopting the surname “Bunker”—though its origin is uncertain, with rumors ranging from the man ahead of them in line to their New York banker. They also learn that their grand home is designed for two future families.

At the party, Sallie is impressed by their wealth and extensive library. She walks through and examines the books, and then the brothers join her. Eng speaks to her directly for the first time, pulling down a copy of Mary Shelley’s Frankenstein and showing it to her. She notes how the library reveals the breadth of their learning and interests. 

Afterward, the brothers entertain their female guests with witty banter and music, demonstrating the polished public personas they have developed during years of exhibition. They also make their intention to marry clear. As the event concludes, Chang directly declares his romantic interest in Addie, while an awkward Eng does the same for Sallie. Despite Sallie’s hesitation, Addie agrees to Chang’s proposal that the four of them meet secretly in two days at an abandoned farm.

Despite Sallie’s reservations about lying and the scandal of courting conjoined twins, Addie persuades her to secretly meet with Chang and Eng. Addie manipulates Sallie, warning that she will end up like their largely housebound mother if she’s too fearful. The sisters sneak away to an abandoned farmhouse to meet the brothers.

There, Chang and Eng recount their past in Siam, describing their impoverished childhood and a summons to appear before King Rama, where they and hundreds of other men were all made to crawl on their bellies toward the throne. It was there where they first felt like “monsters.” They acknowledge, however, that their condition also brought them wealth and travel. After the meeting, Addie pressures a reluctant Sallie to continue seeing the brothers. She argues that it’s recompense for a past event that has limited both their marriage prospects, insisting that a relationship with Eng is Sallie’s best remaining option.

Sallie recounts an “incident” that led to her getting pregnant at age 17. A charming traveling farm-equipment salesman, Tom Fellows, visited her family and, after gaining her father’s trust, proposed a walk to see the property. David gave his permission and withdrew to do paperwork, leaving Sallie alone with Fellows, who sexually took advantage of her age and innocence. Weeks later, after Fellows’s wife appeared and made accusations, Sallie’s pregnancy became visible, causing a family scandal. To hide the shame, her parents sent her away to live with her estranged and unconventional aunt Joan.

At Joan’s remote cabin, Sallie, who was previously spoiled, adapted to a life of hard manual labor, finding unexpected purpose and satisfaction in meaningful physical work. As her pregnancy progressed, she and her non-judgmental aunt bonded. Joan eventually revealed that she had built the cabin with a female partner, Myrtle, who died years ago. She had chosen this life to remain true to herself rather than conform to her family’s expectations.

At her aunt’s cottage, Sallie engaged in a debate about slavery with Joan, who forcefully argued that it was immoral, un-Christian, and corrupting. This challenged Sallie’s ingrained beliefs and prompted her to recall her childhood friendship with Grace, an enslaved girl. She reflected on the painful shift in their relationship when Grace was forced into servitude, forbidden from learning to read, and made to call Sallie “Miss Sarah.” Later, Joan hinted at a family secret: Sallie’s brother Alston was “unexpected,” and David was “decent enough to marry” their mother (55)—implying a premarital pregnancy. 

A few months into her stay, Sallie suffered a fall and had a miscarriage. Joan expertly cared for her without a doctor through the painful and bloody ordeal. Upon returning home, Sallie was met with her parents’ visible relief that she was no longer pregnant. Although the immediate crisis was over, Sallie understood that Joan’s influence had permanently altered her worldview. At the same time, her own tarnished reputation had also ruined Addie’s marriage prospects.

The courtship between Sallie, Addie, and the conjoined twins intensifies, moving from occasional walks to more deliberate encounters. A picnic trip to Wilkesboro exposes them to widespread public gossip and stares, making the group uncomfortable. To deflect, Addie prompts the twins to share their life story. They recount how merchant Robert Hunter and ship captain Abel Coffin paid their mother to let them leave Siam for a five-year tour, though they never returned or saw her again. They describe their journey to the US and their rise to fame as performers. They were exploited by managers who branded them as “The Monster.” Eventually, they decided to take control of their own exhibitions after the Virginia legislature decreed that Coffin, as their manager, was legally their enslaver liable for an exhibition tax. While Chang addresses the sisters, Sallie begins to notice that much of it is practiced speech, while Eng quietly rolls his eyes and acts unsatisfied with the performance.

Upon returning home, the sisters’ father angrily confronts them about the scandalous, unchaperoned outing. Later, he speaks to Sallie alone, expressing his disapproval of her involvement with the “unnatural” twins and offering to help her find a more suitable husband. Sallie confronts him with the long-held implication that her mother was pregnant at the time of their wedding. David admits that he married for “security” and “respectability,” not happiness, and warns Sallie that the road she’s on will only lead to misery.

Ignoring their father’s disapproval, Sallie and Addie meet Chang and Eng for a secret quadrille lesson at an abandoned farm. During the dance, a connection forms between Eng and Sallie, which deepens when he later recites a romantic French poem to her. He points out to her that, even though the public sees them as one, he and his brother are distinct individuals. 

After the lesson, the twins reveal their desire to marry and have children. They explain their unusual proposition: Since they can’t be separated, their wives would have to share a marital bed with both brothers. They describe a technique of mental detachment that they call “alternate mastery” to allow for privacy. Chang also argues that, despite the challenges of such a marriage, it could offer the sisters greater security and opportunity than their diminished prospects otherwise allow. 

When David learns of the rendezvous, he confronts Sallie and Addie. He angrily forbids any union with the twins, citing moral objections, the social scandal, and the prospect of biracial children. He confines his daughters to their property and prohibits them from seeing the brothers again.

As winter approaches, Sallie secretly writes to Eng, admitting her hesitation to marry him because he’s conjoined. Eng replies, insisting that he’s his own person. While Sallie struggles with her father’s disapproval and her own fears, Addie relentlessly pressures her to agree to the marriage, as she can’t wed Chang unless Sallie weds Eng. The situation becomes critical when Charles Harris informs them that the brothers are in Philadelphia, Pennsylvania, planning to undergo a dangerous separation surgery prompted by Sallie’s reluctance.

Determined to stop them, the sisters take $75 from their father’s locked desk drawer without permission and travel to Philadelphia. There, they confront the brothers and convince them to cancel the operation. In this moment, Sallie has a revelation, accepting that Eng’s bond with his brother is an inseparable part of who he is. She takes his hand, committing herself to their future together. Before the sisters leave, Eng gives Sallie a purse with exactly enough money to cover their travel costs. They return home to their father’s fury, and Sallie secretly replaces every bill she had taken from his desk.

After a tense confrontation with their disapproving mother, Addie and Sallie are confronted by their father. Addie declares that she and Sallie are determined to marry Chang and Eng, with or without his consent. She reveals a plan to elope, having already secretly arranged for a minister and sent a message to the twins. A hesitant Sallie feels pressured to agree. However, after receiving a formal letter from Chang and Eng requesting permission to marry his daughters, David unexpectedly relents and gives his blessing. Sallie remains uncertain about why he changed his mind. 

The courtships become official but are carefully supervised, and the family faces immediate backlash from the community, who shun them and spread malicious gossip. During a discussion, Chang is accepting of the risk of having conjoined children, but Eng admits that being joined is sometimes intolerable. The hostility culminates when a mob arrives at the family home with torches, shouting threats and calling the unions an “abomination.” David calmly but firmly confronts the crowd, shaming them until they disperse. While the immediate threat is gone, the community’s quiet disapproval remains.

On the eve of their wedding, Sallie feels anxiety about leaving her family farm and marrying Eng, while Addie looks forward to marrying Chang. Sallie is especially worried about the unique intimacy of a foursome marriage and the public scrutiny it will bring.

On the wedding day, as they get dressed in identical white gowns, Sallie’s anxiety peaks. She becomes emotional and tells Addie that she can’t go through with the ceremony. Addie calms her sister down by advising her to focus only on the next step instead of their entire future. This practical approach steadies Sallie’s nerves. Though still fearful, she agrees to proceed, and the two sisters descend the staircase to meet the waiting brothers for the ceremony.

In a double ceremony at an improvised altar, the two sisters marry Eng and Chang. The pastor acknowledges the union’s unprecedented nature before proceeding with the vows. When no one objects, the ceremony concludes, and the couples are officially married. At the reception, an uneasy Sallie drinks wine to dull her nerves while observing the guests.

Aunt Joan makes a surprise appearance and has a private conversation with Sallie. She advises her to retain her sense of self within the unusual four-person arrangement. Joan also offers Sallie a permanent safe haven should she ever need it again. Before departing, Joan delivers this offer directly to Eng and Chang, telling them that Sallie will always have a place at her table. The evening ends with the two couples performing a practiced dance for the crowd. Afterward, Chang, satisfied that they have given the guests the spectacle they desired, directs the guests to leave.