In modern-day Tokyo, a city packed with dojos, the master of the Isshinryu school, Hachikazu Soudou, is a martial artist of extraordinary skill who wields a style that blends sorcery with combat. Despite his strength, he is powerless to save his childhood friend, who is dying from an illness that modern medicine cannot cure. Grief-stricken and desperate, Soudou is approached by Shizuku Yabusaki, an agent from a government information agency investigating the Dark Age, also known as the Blank Century — a hundred-year period in history for which no records exist. It is rumored that this lost era once possessed knowledge and power far beyond anything available in present-day civilization. Believing that a cure for his friend might lie hidden there, Soudou accepts Shizuku's startling proposal to travel back in time together.

The Isshinryu school is a unique martial lineage passed down through eighty-one generations. Its most critical secret is that each generation's master can pass on his memories and experiences through the art itself, making the style a living archive. Soudou carries a weapon — an axe — that was first crafted by the school's founder using ancient knowledge, a physical testament to the school's deep ties to lost history. However, the records of the 61st master, who lived during the Blank Century, are missing from even the Isshinryu tradition, making the era a complete void of information.

Once Soudou and Shizuku arrive in the Dark Age, everything defies their expectations. The rules of the Isshinryu school's power begin to intensify rapidly, and the "Fist of Lost Words" emerges as a fearsome technique that others in this era dread. As they journey deeper into the past, they head toward the castle of Hinakage, where they find themselves on the verge of a direct confrontation with those who wield the same feared art. Through battles that transcend time, the truths of the Dark Age gradually come to light. Soudou's struggle to hold on to hope and find a way to save his friend drives him forward into a war that is as much about the secrets of history as it is about the strength of his own fists.