The manga is set in 16th century Japan during the final chaotic years of the Sengoku period, a time when powerful warlords vied for control of a fractured nation. The central figure is Kuroda Kanbee Yoshitaka, a man whom Tokugawa Ieyasu, the eventual unifier of Japan, would later credit as the only other person capable of ruling the country. The story follows Kanbee not as a celebrated lord, but as a masterless strategist whose only weapons are his cunning and intelligence, which he uses to overcome enemies far more powerful than himself. His journey begins as a relatively poor retainer of the Kodera clan in Harima province, where his talent for tactics quickly distinguishes him.

As the narrative progresses, Kanbee becomes deeply attracted to the revolutionary unification plans of the powerful warlord Oda Nobunaga. He travels to Gifu Castle to meet him, and it is there that he forms a crucial alliance with Nobunaga's ambitious retainer, Toyotomi Hideyoshi. Kanbee soon becomes Hideyoshi's chief strategist, marking the birth of the "military genius" known as Gunshi Kanbee. One of the most notable narrative arcs involves Kanbee’s imprisonment. When his insistence on allying with the Oda clan makes him a liability to his own lord, he is thrown into a dungeon for over a year. Even from his cell, he devises strategies, sending letters to Nobunaga that outline plans to bring down his captors using tactics drawn from classical Chinese military wisdom. This period of confinement hardens him, transforming him from an idealistic young man into a cool and calculating tactician.

The manga chronicles many of the era's defining military campaigns, including the siege of Miki Castle where starvation becomes a brutal weapon, and the siege of Takamatsu where Kanbee famously uses flooding tactics to force a surrender without a bloody assault. The story follows Kanbee’s rise as he works hand-in-hand with Hideyoshi to subdue the Mori clan and advance toward the unification of Japan. Throughout these events, he cultivates a core group of loyal vassals, known as the Kuroda warriors, and navigates complex relationships with iconic figures such as the brilliant but sickly strategist Takenaka Hanbei. The series also explores Kanbee's personal life, including his marriage to Teru and the coming-of-age of his son, Nagamasa, portraying the human cost and familial bonds behind the legendary battles.