At Naraku's Castle, Kohaku appears before Naraku. Naraku asks Kohaku if can he kill them. Kohaku confirms this with Naraku and calls him 'Master'. Naraku asks questions to Kohaku about Inuyasha, Miroku, Sango, and the others. Kohaku confirms this with Naraku again. As everyone is asleep, Sango wakes up and she remembers that Naraku told her that he made Kohaku forget and he erased all his memories. Sango sits up and she remembers that Naraku demands that she bring him the Tessaiga, Inuyasha's sword, and he will give Kohaku eternal life. Sango tries to reach for the Tessaiga from Inuyasha but stops. Sango wonders what was she doing as she ponders whether or not to steal the Tessaiga. Inuyasha wakes up and asks Sango "What's the matter?" 

Sango lies to Inuyasha about it being nothing as she couldn't sleep and that's all. Inuyasha becomes silent for a moment as he sympathizes with her and tells Sango that he's going to kill Naraku although he feels badly for her. Sango tells Inuyasha that she already told him that Kohaku wasn't himself as her younger brother could never slaughter an entire village and she will deal with him and Naraku at the same time. Kagome secretly wakes up and listens to Inuyasha and Sango's conversation. Inuyasha questions Sango if it's as simple as Kohaku looking exactly the same and wonders if can one hate and forget so easily. Sango tells Inuyasha to not underestimate her as she's a demon slayer and she won't fall for dirty tricks. Inuyasha tells Sango that if he were in her position he probably couldn't do it. Sango becomes silent. Kagome thinks to herself that it looks exactly the same so Inuyasha can't hate or forget as he is speaking of Kikyō. Miroku then wakes up and calls out to Inuyasha as he warns everyone that they are surrounded. Indeed, countless demons attack, led by a Kohaku deprived of his memories and will by Naraku. Katsuki fights the demons, and then Kohaku. 

To everyone's surprise, Kohaku then talks, in a voice devoid of emotion he says that indeed he deserves to die since he killed so many people, including his family and friends. He stabs himself to try and extract the Jewel shard from his back, but Sango yells at him not to do it and uses her Hiraikotsu to send the Tessaiga flying in the air. The Tessaiga falls back into its normal form of an old katana. Kohaku relents and flees with the demons to Naraku. Sango, still confused, sees the Tessaiga stuck in the ground very close to herself and wonders what she should do about the trade with Naraku. Suddenly Sango takes the Tessaiga and calls Kirara to fly after Kohaku, which shocks the others. While her friends follow her by scent, Sango arrives at Naraku's castle and calls for Kohaku when she hears Naraku's voice telling her to put down her weapon. Sango confronts Naraku and demands the return of Kohaku as she's even ready to throw her Hiraikotsu away to see Naraku. Naraku reveals Kohaku to Sango from the castle's curtain but puts the blame on the Sacred Jewel for all the evil that happened. Enraged, Sango attacks, throwing the Tessaiga at Naraku and using her own concealed blade to slash him. Naraku reveals himself to have borrowed the appearance of the young lord of the castle after killing him. Sango attacks again, but her blade breaks, leaving her unarmed and wounded and she takes a hit from Tessaiga. Kirara attacks Naraku, but to no avail, as Miasma defeats her. Sango worries about Kirara. Now Sango too is attacked by Miasma and lies exhausted on the ground. Naraku then asks Sango to serve him but when she refuses, he has Kohaku come and attack her. Kohaku commences on repeatedly attacking her with his Kusarigama and wounds her severely, yet Sango continues to resist Naraku's offer to serve him. Suddenly, Naraku gives Sango her Hiraikotsu to fight Kohaku, but she realizes that there's just no way she can kill her own brother, and renounces to pick her weapon then runs towards him smiling and collapsing halfway. All of a sudden Kohaku's gaze gets an expression again and he recognizes Sango, Inuyasha, and his friends have arrived at that scene. They see Kohaku fall to the ground, horrified at his own deed. Kagome goes to Sango while Naraku wonders why she couldn't kill Kohaku but he seems unable to comprehend how one could love someone more than their own lives. Sango that Kohaku shouldn't be more important than yourself. As Sango lays on Kagome's lap, recovering, she only sees that Kohaku is slowly regaining consciousness and even seems optimistic. Miroku realizes to Inuyasha that this is exactly what happened with him 50 years ago. Inuyasha questions Miroku. Kagome realizes that Naraku tried to make Sango hate Kohaku so that she would kill him then the Sacred Jewel in his back would become more evil as it's the same trick he used to make Kikyō and Inuyasha hate each other and fight as. Kagome angrily says she can't forgive Naraku and she will never forgive him for this. Kagome thinks to herself that Kikyō died so that the Sacred Jewel would remain dormant and in the same way Sango refused to fight Kohaku. Kagome says Naraku's such a fool as there's no way Sango would have done it as she is a kind sister who loves her brother.

Angered, Naraku surrounds them with miasma. Sango remembers a flashback with Kohaku about their father before she slowly gets up. Kagome becomes worried about Sango. Inuyasha demands that she keep still as he's got a lot of things to say to her so she doesn't dare die. Sango thinks to herself that it's her fault but before she gives Kohaku her breathing mask. This act of selfless love purifies the Sacred Jewel shard embedded in Kohaku's back. Kohaku's Sacred Jewel shard reminds Kagome that she can locate Naraku, even in the miasma, by the other Jewel shards in his possession. Kagome tells Naraku to prepare to die before she shoots a sacred arrow at him. Due to Kagome's spiritual powers, this creates a large explosion and Naraku stands there with one arm shot off and is surprised that the miasma has also been purified. Kagome tells Naraku that he is so despicable. Naraku wonders about Kagome and is her power and with hardly any effort she can purge the miasma the poison vapors as it's the power that belonged to Kikyō and it can't be. Kagome tells Naraku that she won't let him go even if he begs forgiveness, Izuku gets annoyed so he and Kakarot shoot a double sacred arrow at him, impaling his chest. Naraku realizes that this is more effective sacred arrows and that means these new members were a threat more than Kikyo. The sacred arrows ripped Naraku apart from his head before a violent wind arose. The Alpha sheltered their mates and when it died down, Naraku flees first and Sango realizes that her sleeping brother Kohaku is also being taken and repeatedly calls out to him. Naraku's castle disappears.

As the sun rises, Sango tries to leave her friends, feeling she's unworthy of their trust and might end up betraying them again, but they all say it's alright and continue to wish for her to stay in their group and fight Naraku. Upon hearing this Sango breaks down into tears and falls onto Izumi's lap hugging her. As she cries Izumi consoles her that everything will be alright, and they will both defeat Naraku and save her brother Kohaku. The next morning, they wake up in the open field and realize the castle is just an illusion. Miroku tells Inuyasha the Tessaiga, who notices it nearby. Kagome realizes that there are no Sacred Jewel shards around, so Naraku is no longer there. Despite her injuries, Sango wants to leave because she feels guilty about her actions and might end up betraying them again. However, her friends say it's alright and continue to wish for her to stay and that they will all save Kohaku and fight Naraku together. When Inuyasha even says she should stay, Sango breaks down into tears and falls onto Izumi's lap hugging her. As Sango cries Izumi consoles her that everything will be alright, and they will both defeat Naraku and save her brother Kohaku.