In the isolated kingdom of Banka Koku, which has long closed its borders to preserve its own culture, a young girl named Getsu Ei is born with emerald-green eyes—a trait considered heretical. To hide her origin and survive, she conceals her eyes beneath long bangs, dresses as a boy, and lives quietly on the fringes of society. She supports herself with a rare and mysterious healing art called "fragrance therapy," which uses specially blended scents to cure physical and mental ailments.

One day, Getsu Ei is abruptly seized and brought to the imperial palace, where she is ordered to treat the severe insomnia of a man with a strikingly handsome face marred by deep dark circles. That man is the crown prince, the absolute authority of the nation. Forced into the role of a court physician, she must now navigate the all-male palace environment while keeping both her gender and her true identity secret. Her unorthodox methods and grassroots determination gradually prove effective, drawing the attention of nobles and courtiers who begin to seek her help for ailments conventional medicine cannot cure.

As her reputation spreads, Getsu Ei gains a disciple named Banri, who soon discovers her secret, leading to a strained but evolving relationship. A major arc sees her falsely accused of attempting to poison the public through a scented tea called "ikōcha," which had become a popular commodity. Thrown into prison, she must uncover the truth behind the conspiracy to save her own life and protect the future of fragrance therapy itself. In another arc, the kingdom opens slightly to the outside world, and Getsu Ei is tasked with healing a foreign consort named Ahi who has withdrawn into silent grief, prompting a journey beyond the palace walls to find the scents of her homeland. Through each challenge, Getsu Ei's unconventional skills, stubborn courage, and ability to connect with others through fragrance begin to transform not only the court but also the closed-off kingdom itself.