So'unga, a legendary sword that belonged to InuYasha's father, is unleashed from its centuries-old seal and seeks to destroy the Earth and all life on it. Thus, InuYasha must join forces with his brother, Sesshomaru, and destroy the sword before it destroys the world.
MOVIE OVERVIEW:
The film opens with events that happened at InuYasha's birth. The suitor Takemaru kills Izayoi just before InuYasha's father arrives. InuYasha's father, still injured from his battle with Ryuukotsusei, arrives and resurrects Izayoi, telling her to escape with their son, whom he names as InuYasha and gives her the red Fire Rat Armor to protect them from the burning palace. Presumably, InuYasha's father died with Takemaru when the entire complex crashed and burned down as the two dueled inside the palace.
In the present era, Kagome's Grandfather comes by with a sword which he calls a 'national treasure'; as Grandfather is about to put it in the storehouse for "safe keeping", Kagome's younger brother goes over to the sword, and it speaks. The sword is So'unga, the sword of hell, and it possesses a connection to InuYasha's father. It tried to possess InuYasha, but InuYasha tried his best to resist being controlled and finally brings the sword back to the past, but in the process unleashes the deadly powers the sword holds on an evacuated village.
Sesshomaru is also alerted of the presence of So'unga, and he follows the scent to find that InuYasha is in possession of the sword. The two of them duel and at a crucial moment, Kagome saves the day and forces the So'unga to release InuYasha from its grasp. It finds a new demon host and recovers Sesshomaru's lost left arm, then revives Takemaru and subsequently possess him, aiming to take revenge on InuYasha and Sesshomaru. Both Inuyasha and Sesshomaru have reasons to defeat Takemaru so they go into battle. During the battle Kagome and Rin are kidnapped. Both of the brothers take off to protect the girls. Inuyasha manages to defeat Takemaru just as Takemaru realizes Inuyasha's mother wanted him to live and that he loved her. Inuyasha used his human traits to defeat Takemaru.
So'unga was the third sword that InuYasha's father possessed. Individually, it was stronger than either the Tenseiga or the Tetsuseiga. Sesshomaru wields the Tenseiga, and InuYasha wields the Tetsuseiga, as they dueled they managed to combine their attacks and destroy sounga, causing the sword to fall into the pit of hell. At the end, Inuyasha and his brother see their father as he thanks them for discovering how to destroy the sword. After the credits, Inuyasha and Kagome both lay on a hill, and then Kagome tells Inuyasha that she thought Sesshomaru didn't like people, and Inuyasha replied that he didn't care at all. Meanwhile Kagome remembered something: Inuyasha's enchanted beads. Kagome ordered Inuyasha to sit up and close his eyes. While his eyes were closed, Kagome placed his enchanted beads around his neck. Suddenly Inuyasha realised that the "nice" surprise was actually the enchanted beads! Inuyasha tells Kagome to take it off, but Kagome refused to because she didn't want Inuyasha running off again, and an argument was going on and in the end, Kagome tells Inuyasha to sit very sweetly and that causes Inuyasha to fall on the ground and the screen goes black, ending the movie.
