The novel is set in an alternate United States governed as a constitutional monarchy. It opens with the funeral of King George IV, who has died of lung cancer at fifty. His eldest daughter, twenty-two-year-old Beatrice, becomes America's first queen.
Six weeks later, Beatrice returns to Washington Palace still grieving and haunted by guilt. She believes her father's fatal episode was triggered by her confession that she loved Connor Markham, a member of the Revere Guards, the royal family's elite bodyguard corps, and wanted to renounce the throne. Robert Standish, the Lord Chamberlain, the senior official who manages the royal household, pressures her to marry her fiancé, Theodore "Teddy" Eaton, the son of the Duke of Boston. Robert argues that a royal wedding would unify the unstable nation, and Queen Adelaide, Beatrice's mother, agrees. Though Beatrice had planned to call off the engagement, she relents, sensing that a young, single, female monarch is exactly what makes America anxious.
When Beatrice tells Connor the wedding is real, he is devastated. They share a desperate final kiss, and he resigns. After he leaves, Beatrice sits in her father's chair for the first time.
Princess Samantha, Beatrice's younger sister, learns at the Royal Potomac Races that the wedding is going forward. Teddy explains that his family's fortune has collapsed and marrying into the royal family is the only way to save the duchy of Boston. Furious, Sam storms away. At a museum gala, she meets Lord Marshall Davis, heir to the dukedom of Orange, a former independent West Coast kingdom whose founders were formerly enslaved people. Sam proposes a fake relationship to make their respective exes jealous: Sam wants to provoke Teddy, and Marshall hopes to win back the actress Kelsey Brooke. Marshall warns Sam that as a person of color publicly dating a royal, he will bear the brunt of racist media attention, but he agrees.
Nina Gonzalez, Sam's best friend and Prince Jefferson's ex-girlfriend, has been rebuilding her life at King's College after the media frenzy surrounding her relationship with the prince. At school, Nina is paired on a journalism project with Ethan Beckett, Jeff's best friend. Unbeknownst to Nina, Daphne Deighton, Jeff's scheming ex-girlfriend, has sent Ethan to monitor Nina and keep her away from the prince, promising him a future earldom, a noble title and estate, in exchange. Despite these origins, Ethan and Nina develop a genuine connection, sharing vulnerable confessions and a kiss in her dorm room one night before Ethan abruptly leaves.
Daphne is managing multiple crises. Her former best friend, Himari Mariko, has woken from a coma caused by a fall at Sam and Jeff's graduation party nearly a year earlier. The fall was Daphne's fault: She had drugged Himari's drink after Himari caught Daphne with Ethan and threatened to tell Jeff. Himari initially remembers nothing. Daphne also needs Jeff back, so she tips off a tabloid reporter about Nina and Ethan, ensuring the reporter calls Jeff directly for comment. Jeff feels betrayed, stops speaking to Ethan, and turns to Daphne as his most loyal remaining ally.
Beatrice's relationship with Teddy deepens unexpectedly. During a chance encounter in a palace hallway, she pulls him into a linen closet to hide from her mother. Teddy gives her a butterscotch brownie he remembered she once mentioned, and she cries for the first time since her father's death. He proposes a pact: They will always be honest with each other. During a later visit to Walthorpe, the Eaton family estate, Beatrice discovers documents revealing the family's financial ruin. She promises to assume the debts and buy back their sold Nantucket house, and they pledge to face the future together. That evening she relaxes for the first time, playing video games with Teddy's brothers and adopting a puppy she names Franklin. She kisses Teddy; he stops her because she is drunk but stays the night when she confesses her nightmares.
Robert continues to undermine Beatrice, blocking her from Congress and insisting she cannot govern until she is married and crowned. When Congress fails to invite her to its closing session, Beatrice drives to Columbia House, where Congress meets, in full ceremonial regalia, only to find Robert presiding in her place. He tells her a young, single woman makes the nation feel unsettled. Humiliated, Beatrice flies to Nantucket, where Teddy reassures her.
Sam and Marshall's fake relationship escalates after poolside photos from a palace party go viral. Queen Adelaide and Robert mandate that they double down publicly. In Orange for Accession Day, the duchy's annual celebration, Sam and Marshall share vulnerable truths. Marshall confides about his dyslexia and the family pressure behind his party-boy image, while Sam shares her abandoned dream of becoming a lawyer. She reaches for his hand in a private moment but later sees him dancing with Kelsey and is gutted. She realizes she does not love Teddy. She loves Marshall, but believes she has lost him.
At their father's grave, Sam and Beatrice reconcile. Sam reveals she knows about Connor, having found his unsent wedding invitation in a secret desk drawer. Beatrice breaks down, confessing her guilt over their father's death, and Sam insists he died of cancer, not shock. Beatrice then admits she is falling for Teddy. Sam gives her genuine blessing and confides her feelings for Marshall.
Himari's memories return, and she confronts Daphne. Desperate, Daphne blackmails Beatrice, threatening to expose the queen's secret relationship with Connor unless Beatrice appoints Himari's parents as ambassadors to Japan, removing Himari from the country. Beatrice complies but warns Daphne never to try again.
On the wedding morning, Connor appears at the palace, having received the invitation Sam impulsively mailed. Sam, seeing Beatrice's anguish, activates the palace emergency alarm, slamming steel-reinforced doors throughout the building and halting the ceremony. Sealed in the Brides' Room, Beatrice faces Connor one last time. He asks her to run away with him. She tells him she loves Teddy and no longer needs rescuing. They share a tender goodbye, each promising to keep safe the piece of the other's heart they once held.
During the lockdown, Daphne finds Ethan and confesses her love. He tells her it is too late: He loved her for years, but her repeated rejection exhausted his feelings, and he has fallen for Nina. After the lockdown lifts, Daphne reveals to Nina that she orchestrated Ethan's pursuit from the beginning. Nina is shaken but tells Daphne she will end up alone.
Beatrice fires Robert and postpones the wedding indefinitely, declaring she must prove she can rule alone. She asks Sam to undertake the summer royal tour in her place. Marshall, having witnessed an innocent goodbye between Sam and Teddy, storms off believing he was used. Sam chases him down, confesses everything, and tells him he is the one she wants. They agree to be together.
Nina tells Ethan she believes his feelings are genuine but cannot yet trust him. She asks for time apart. Jeff visits Daphne and gives her his family signet ring, signaling their official reunion. Daphne recognizes she has achieved everything she schemed for and that it means nothing. She slides on the ring, her smile frozen in place.
Beatrice tells Teddy she wants to prove she can govern independently before they marry, promising it will happen on their own terms. She steps alone onto the palace balcony to face the crowds, not as a bride, but as a sovereign. Sam invites Nina to join her on the royal tour, and the two friends plan a summer of travel and self-discovery.