Izumi Tanaka is an eighteen-year-old Japanese American senior living in the small, predominantly white town of Mount Shasta, California, with her single mother, Hanako, a biology professor. Izumi has never known her father; Hanako always claimed he was an anonymous Japanese exchange student she met at Harvard. One afternoon, Izumi's best friend Noora discovers a love poem in Hanako's bedroom signed by "Makoto" and dated 2003, the year Izumi was born. A Google search links the name to Makotonomiya Toshihito, the Crown Prince of Japan and heir to the Chrysanthemum Throne, Japan's imperial seat. Photos confirm an unmistakable resemblance. When confronted, Hanako admits she knew the Crown Prince was Izumi's father but kept it secret to shield her from the constricting life of imperial women. After a painful argument, Izumi sends the Crown Prince an introductory letter through a mutual friend.
Nearly two weeks pass with no response before Japanese tabloid reporters appear on Izumi's lawn. The Japanese Ambassador delivers a handwritten invitation from the Crown Prince requesting Izumi visit him at Tōgū Palace, his official residence in Tokyo. Despite her mother's apprehension, Izumi accepts.
Izumi flies to Tokyo but neglects the informational dossier from the Imperial Household Agency, which manages the royal family's affairs. At the airport, she meets Akio Kobayashi, her twenty-year-old personal imperial guard, who is attractive but cold and impatient. She fails to recognize him because she skipped the dossier, creating immediate friction. Mr. Fuchigami, the East Palace Chamberlain and senior household official, escorts Izumi by motorcade through the city. For the first time, Izumi feels the powerful experience of blending into a population that looks like her.
At the palace, Izumi meets her lady-in-waiting, Mariko, an expert in court etiquette. In his office, Izumi and the Crown Prince share an awkward first encounter that breaks when he admits his hands are shaking. They walk the gardens, where he reveals he chose the purple iris as her floral emblem. He tells her that if he had known about her, he would have found a way to be in her life. Fireworks erupt over Tokyo to welcome the new princess.
That evening, a formal dinner introduces Izumi to the extended imperial family. Her second cousin Yoshi, a charismatic and irreverent prince, befriends her and guides her through the meal. The twin princesses Akiko and Noriko, Izumi's first cousins, are perfectly poised and subtly hostile. Over the following days, Izumi undergoes grueling etiquette lessons, language study, and cultural training. Akio assigns her the security code name "Radish" after the tabloids co-opt her original name.
After the twins provoke her at a palace event, Izumi accepts Yoshi's invitation to sneak off imperial grounds. They bond over shared feelings of not belonging. After getting drunk and locking herself in a dumpster cage outside a karaoke bar, Izumi is cornered by a stranger before Akio rescues her, revealing he placed a tracker on her phone. In the car, they begin opening up: Akio confesses he is unsure he is meant to be a guard, and Izumi admits the same about being a princess.
Their relationship deepens. On a free day, Izumi redirects the imperial car to Akio's parents' home so he can deliver his mother's medication. She learns his mother has early onset dementia and discovers Akio's childhood dream of becoming a pilot. She quietly arranges for his mother to receive care at the imperial hospital.
Before Prime Minister Adachi's wedding, Izumi and Akio share a slow dance in the palace living room, but Akio pulls away before they can kiss. At the wedding, the twins set Izumi up by whispering that her father failed to mention the PM's estranged sister in his toast. Taking the bait, Izumi raises the topic with the PM, who erupts in fury. As she flees, the twins call her
gaijin, a derogatory term for foreigner. In the car, Akio explicitly rejects any romantic possibility, calling their dance a mistake: A relationship between a princess and a guard is taboo.
Izumi's father announces he must leave for sixteen days to fill in for the ailing emperor and asks Izumi to travel to Kyoto. There, she throws herself into calligraphy and cultural study. A late-night session with Mariko, who reveals her own experience as a cultural outsider, forges a genuine friendship. On the palace grounds, Akio introduces the concept of
ninjō, human emotion that conflicts with duty. They set aside their titles and share their first kiss, then begin exchanging secret
waka, short classical Japanese poems. The people of Kyoto line the streets with paper lanterns to welcome Izumi, and she is overwhelmed with acceptance.
Back in Tokyo, Izumi and her father reconcile. She gives him a calligraphy scroll; he gives her a researched genealogy of her mother's family, including photographs of her maternal grandmother, a picture bride who emigrated to marry a man she knew only from a photograph. At the emperor's birthday, the emperor bluntly states Izumi's parents should marry and produce a male heir, but Izumi argues for gender equality in the line of succession and earns the empress's approval. The empress requests the entire family join on the balcony to greet forty-five thousand well-wishers. In an empty ballroom afterward, Akio confesses he is "heartsick" for Izumi, and they kiss.
The next morning, a tabloid exposé reveals photos of Izumi and Akio from multiple locations, attributed to a "palace insider." Akio is no longer with the Imperial Guard, and his phone is disconnected. Izumi asks Yoshi to deliver a letter requesting Akio meet her, but he never appears. Heartbroken, she flies home without telling her father goodbye.
After a week of grief in Mount Shasta, the Crown Prince arrives unannounced for Izumi's graduation. Over three days, they hike, eat at the local diner, and bond. He and Hanako reconnect with obvious affection. Izumi confesses her fear that she will never be good enough for Japan; her father responds that she belongs with him. After watching his car leave following graduation, Izumi has an epiphany: She does not have an American half and a Japanese half. She is a whole person. She and her mother chase the motorcade and declare they are going to Japan.
Back in Tokyo, Izumi gives an unprecedented exclusive interview with
Women Now!, speaking candidly about her identity struggles and relationship with Akio. The article generates an enormous positive response. When the twins deny leaking the story, Izumi confronts Yoshi, who admits he sold the photos to fund his independence from the imperial family. Devastated, Izumi cuts ties and, as a deliberate act of retribution, hires Yoshi's own guard, Reina, as her new head of security.
Among the gifts left at the palace gates, Izumi finds a handmade rainbow key chain with her name carved into it and a
waka poem. She recognizes it as Akio's, a reference to a story she told him about never finding her name on key chains as a child. She races to the gate and finds Akio waiting. He reveals he was never fired; he quit voluntarily to enlist in the Air Self-Defense Force, Japan's air force, fulfilling his childhood dream. He explains that he stayed away because he believed he had ruined her life, and that Yoshi never delivered her letter. Akio tells Izumi she is "a world unto herself." They dance without music as night falls, and Izumi reflects that while she does not know what happens next, the present moment is enough.