When the young noble Bard Cornelius falls ill, a healer delivers shocking news to his father: the boy's body now houses three separate souls. Alongside Bard's own consciousness exist the spirits of two men from another world who died and were reincarnated into him. One is Oka Sanai Sadatoshi, a cunning and miserly samurai warrior from Japan's Sengoku period. The other is Masaharu Oka, a modern high school otaku whose obsession with animal ears was cut short by a traffic accident on the day of his university entrance exam. As Bard grows, the distinct knowledge and personalities of his two internal passengers begin to surface, granting him an extraordinary advantage in the fantasy kingdom of Mauricia.

From childhood, Bard demonstrates abilities no normal noble's son should possess. The samurai Sanai contributes brilliant military tactics and a relentless drive for wealth, often urging Bard toward profitable and dangerous opportunities. Masaharu provides encyclopedic knowledge from the modern world, allowing Bard to introduce revolutionary concepts like refining sugar from beets and creating gold ornaments through electroplating. These innovations make Bard and his family's territory incredibly prosperous, but also attract unwanted attention from spies and rival factions seeking to steal his secrets. When he foils one such espionage attempt, Bard is sent to the royal knight academy, setting him on a path far beyond simple commerce.

The young noble's reputation soars after he successfully treats the second princess Rachel during a cholera outbreak using Masaharu's medical knowledge, teaching the court crucial sanitation methods to prevent further infection. In gratitude, the king awards Bard a noble title, making him Baron Severn. He continues to accumulate achievements by blending Sanai's battlefield brilliance with Masaharu's strategic insights, defeating enemy forces that threaten his homeland. However, these successes also begin to unravel deeper secrets about his own birth. Bard learns that his mother, Maggot, is actually Princess Margaret of the fallen Tristvis royal family, making him the legitimate first heir to a lost throne. Faced with this revelation, Bard resolves to relinquish his current position and embark on a monumental campaign to reclaim Tristvis.

Bard's journey takes him to the neighboring Kingdom of Sanjuan on a diplomatic mission for King Welkin, where he becomes entangled in a violent succession crisis triggered by the death of the crown prince. Using his unique blend of skills, he forges alliances with childhood friends and Prince Franco while providing the Sanjuan navy with technological advances such as the compass and preventive measures against scurvy. These contributions earn him a new territory and the title Viscount of Antorium. When the neighboring Haurelia Empire invades his new lands, Bard holds the line against overwhelming forces until reinforcements arrive, repelling the invasion and earning promotion to Margrave of Antorium. As the story progresses, the three minds within Bard must learn to cooperate more deeply, navigating political intrigue, large-scale warfare, and the challenge of sharing a single body while each pursues their own goals.