Sanosuke's friend has died from an opium overdose. However, Kenshin and Sanosuke are perplexed at how he even got his hands on the drug, since opium is banned and too expensive for the average person to afford. Then a mysterious woman, Megumi Takani, appears before them. Kenshin and the others fight off Kanryu Takeda's private army who is trying to capture Megumi. However, Megumi is carrying opium...

Hyottoko, a member of the Oniwaban group, raids Kamiya Dojo, where Megumi is hiding. He unleashes his special attack "Fire Breath," in which he breathes fire from his mouth, onto Kenshin and Sanosuke. Anticipating the defeat of Kenshin and the others at the appearance of the powerful middle-ranked spy, Megumi tries to flee the scene, but Kaoru stops her and says "He's fighting for you," telling her to believe in Kenshin's strength.

The pursuers from the Oniwaban Group haven't made an appearance for over a week, and during this time Megumi familiarized herself with the Kamiya Dojo group. Sanosuke, however, was still unsettled and feeling frustrated knowing that Megumi was the one who made the opium that caused his friend's death. Then one day, Megumi suddenly disappears. In a letter they found from her, it stated that she had left of her own volition, but Kenshin and the others found this to be unnatural. They decide to head to Kanryu Takeda's residence, the man who was sending spies after her.

Kenshin, Sanosuke, and Yahiko scramble to take back Megumi from the Kanryu residence. They are confronted by Han'nya, a spy of the Oniwaban Group. He trifles Kenshin with his martial arts skills and his "Arm-extension Spell", but Kenshin successfully conquers him when he uncovers the secret of the technique during the fight. Just when it looked like the battle was up, Han'nya removes his mask and reveals his startling true face.

At last, Kenshin faces off against Aoshi Shinomori, the head of the Oniwaban Group. In the Bakumatsu Era, the battle-obsessed "Oniwaban group" lost their meaning of existence after surrendering the castle they served without even being able to fight. Aoshi is unable to bear this regret and is in earnest pursuit of the title of the strongest warrior. As a former Revolutionary warrior, Kenshin cannot tolerate conflict without morality. Two men who made killing their profession in the Bakumatsu Era clash against one another, putting their pride on the line.

In order to kill Kenshin, Kanryu uses his Gatling gun and rains a flurry of bullets at him, uncaring that the Oniwaban Group that was under his command would also be hit by this attack. In the face of this evil act, the Oniwaban Group make full use of their individual abilities to counter the Gatling gun. At this moment, theirs and Kenshin's purpose become one. Kenshin and the Oniwaban group team up to challenge the overwhelming power of a modern weapon.