Following Inu no Taishou's death, his swords Tenseiga and Tessaiga were passed on to his sons, Sesshoumaru and Inuyasha respectively. However, there exists one more blade. During a visit to his friend Kagome Higurashi in modern times, Inuyasha encounters Sounga—his father's third sword. Found by one of Kagome's ancestors inside the old well that connects the past to the future, Sounga has been kept for years as a treasured artifact at the Higurashi shrine. Nevertheless, it soon reveals itself to harbor an evil spirit with a terrible urge to kill.

While struggling to subdue its calamitous nature, Inuyasha manages to bring Sounga back to the feudal era where he must confront his brother Sesshoumaru, who has always desired to possess it. But when they lose track of the sword during their fight, the siblings engage in a fervent competition to find it, still unaware of its secret and the legacy that Inu no Taishou has left for them.