"Parallel World Pharmacy" centers on Kanji Yakutani, a brilliant pharmacologist in modern Japan who dies from overwork and is reincarnated into a medieval European-style world as Falma de Médicis, the son of a noble family. In this new world, medical knowledge is rudimentary, and diseases that are easily treatable in his previous life are often fatal. Falma retains his memories and expertise, allowing him to revolutionize healthcare using his advanced pharmaceutical knowledge.

The narrative explores Falma’s efforts to establish a pharmacy that provides affordable and effective treatments to the common people, challenging the elitist and often corrupt medical practices of the nobility. His actions draw attention from various factions, including the royal family, religious authorities, and rival nobles, who either support or oppose his innovations. Falma’s dual identity as a noble and a healer creates tension, as he navigates political intrigue while striving to improve public health.

Key characters include Falma’s adoptive father, Bruno de Médicis, a respected noble who supports his endeavors; Charlotte, a young girl who becomes Falma’s first patient and later his assistant; and Elisabeth, a royal princess who takes an interest in Falma’s work. The series balances Falma’s scientific pursuits with his personal growth, as he learns to reconcile his past life’s knowledge with the realities of his new world. Themes of ethics, social inequality, and the transformative power of science are central to the story, as Falma’s pharmacy becomes a symbol of hope and progress in a society plagued by ignorance and disease.