Shingen Takeda, leader of the Takeda clan, is gravely wounded while sieging a rival castle. Takeda leaves instructions for his generals to keep his death a secret for at least three years, and to refrain from further conquests, in order to preserve his clan. Takeda soon dies, and the generals manage to keep it a secret by employing a common thief, who bears an uncanny resemblance to Takeda, to impersonate him at public functions. The thief serves as Takeda’s “kagemusha,” or shadow decoy. The Kagemusha is able to fool the retinue at his mansion, as well as rival warlords Nobunaga Oda and Ieyasu Tokugawa. While Nobunaga and Tokugawa suspect that Takeda may have actually died, the appearance of the Kagemusha intimidates them and keeps them from attempting any moves against Takeda.

The Kagemusha is tested in his role by Katsuyori, Takeda’s son. Katsuyori resents the Kagemusha for preventing him from taking full control of the tribe until the three years are up, as well as the generals for enforcing his late father’s wishes to refrain from conquest. Katsuyori desires to make a name for himself and emerge from his father’s shadow. When the three years are over half complete, the Kagemusha makes a fatal error and attempts to mount the late lord’s horse. The horse bucks him, and when he is examined, he is found out to be a fraud. The generals have no choice but to expel the Kagemusha and accept Katsuyori as clan leader.

Katsuyori leads an ill-advised assault against Nobunaga and Tokugawa. The opposing forces have superior firepower and easily destroy Katsuyori’s forces. The Kagemusha, who has become increasingly affectionate and loyal towards his late lord, follows the army to the battlefield and watches the massacre. Finally, even after Katsuyori and his generals have fled, the Kagemusha takes a spear and makes a hopeless charge against Nobunaga and Tokugawa.