During the tumultuous Sengoku period in 1571, the powerful daimyō of the Takeda clan, Takeda Shingen, encounters a thief whose life was spared from crucifixion by his brother Nobukado, due to the remarkable resemblance the thief bears to Shingen. A strategic decision follows—the brothers agree that the thief can serve as a kagemusha, or political decoy. As the Takeda army is engaged in a siege against a castle governed by Tokugawa Ieyasu, Shingen is tragically shot while enraptured by the haunting sound of a flute from the enemy camp. In a desperate move, he orders a retreat and instructs his generals to conceal his death for a mere three years before he ultimately succumbs to his injury. Meanwhile, rivals including Oda Nobunaga, Tokugawa Ieyasu, and Uesugi Kenshin begin to ponder the ramifications of Shingen's withdrawal, blissfully ignorant of his demise.
Nobukado introduces the thief to Shingen's generals with the proposal to have him impersonate Shingen continuously. Initially unaware of the gravity of Shingen’s condition, the thief inadvertently discovers the warlord’s preserved body encased in a large jar, mistakenly presuming it to be a treasure trove. As mistrust arises among the generals, they decide to release the impostor. In a turn of events, the jar is carelessly dropped into the depths of Lake Suwa, witnessed by spies from Tokugawa and Oda forces who quickly suspect that Shingen may have passed away. However, the clever thief, eavesdropping on the spies, returns to the Takeda army with an offer to continue serving as kagemusha. The Takeda clan spins a tale, claiming that they were merely performing a ritual of sake for the lake's god, and the spies are ultimately convinced of the deception by the thief’s performance.
Returning to the Takeda stronghold, the kagemusha adeptly fools Shingen’s retinue through imitation of the late warlord's mannerisms while increasingly learning about him. When faced with a clan meeting, Nobukado advises the kagemusha to remain silent until a consensus is reached among the generals. The kagemusha would then simply agree with their plans and dismiss the council. However, Shingen's son Katsuyori, frustrated with the imposed three-year charade that postpones his claim to leadership, tests the kagemusha in front of the council where many still remain oblivious to Shingen's demise. The kagemusha, rising to the occasion, provides answers in Shingen's characteristic style, leaving the generals even more impressed.
By 1573, Nobunaga mobilizes his forces against Azai Nagamasa, continuing his campaign to secure Kyoto amid rising opposition. As the Tokugawa and Oda forces strike at the Takeda, Katsuyori disregards the counsel of his generals and launches a counter-attack. In a gripping moment, the kagemusha is compelled to lead the troops in the Battle of Takatenjin, where his charismatic leadership inspires a victorious outcome. However, caught in a surge of overconfidence, the kagemusha attempts to ride Shingen’s notoriously unpredictable horse and is unceremoniously thrown off. Observers quickly recognize he lacks Shingen's battle scars, and he is subsequently unmasked as an impostor, leading to his disgrace and Katsuyori's ascendancy to clan leadership. Sensing an opportunity in the Takeda clan's weakened state, the Oda and Tokugawa forces boldly launch a full-scale offensive into Takeda territory.
By 1575, with Katsuyori firmly at the helm of the Takeda army, he spearheads a counter-assault against Nobunaga at Nagashino. Despite the bravery displayed in their attack, multiple waves of Takeda cavalry and infantry face devastation from the relentless musket fire of the Oda arquebusiers, leading to the near annihilation of the Takeda forces. Following closely behind, the kagemusha, driven by desperation, seizes a spear and charges toward the Oda lines, only to be struck down. Mortally wounded, he struggles to recover the distinguished fūrinkazan banner that has fallen into a turbulent river, but ultimately succumbs to his injuries, his body swept away by the current.
