Inside the Black Pyramid, Mumm-Ra is tired of the Mutants' consistent failures to destroy the ThunderCats. Slithe claims that theSword of Omens is to blame as it keeps on growing stronger each time. Mumm-Ra then summons Hachiman, an ancient samurai warrior and possessor of the mighty sword, the Thunder-Cutter, and convinces him that the ThunderCats are evil. Sworn to the code of Bushido, Hachiman sets out to seek and destroy the leader of the ThunderCats, Lion-O.
At Cats Lair, the ThunderCats are explaining to Nayda how their new "Mutant Attack Warning System" works. Panthrosays that once Lion-O has installed it, the system would help to protect the Warrior Maidens' Treetop Kingdom from intruders. Lion-O and Nayda set off for her home but since the normal route would take them three days to reach, Nayda suggests a shortcut across the "Four Day Drop".
The two soon reach the Four Day Drop and Lion-O begins to cross the long log spanning the abyss. As he reaches mid-way across it, Hachiman starts to cross it from the other end and both meet in the middle. With neither of them willing to back down, they start cutting the log using their swords until one of them concedes defeat. The old log breaks and they both fall down the chasm but Nayda rescues them by firing an arrow with a rope attached to it which they grab and climb up.
Hachiman, still not aware of the identity of Lion-O, says that he is searching for lord Lion-O. The young lord of the ThunderCats reveals that he indeed is the Lion-O that Hachiman is searching for and the two get ready to engage in a duel. However, neither the Thunder-Cutter nor the Sword of Omens agree to draw and both Lion-O and Hachiman realize that the duel is wrong. Hachiman then figures out that he had been tricked by Mumm-Ra and befriends Lion-O. The samurai then cuts down another tree with his sword and it falls across the chasm to replace the one that they broke.
Meanwhile, Slithe and Monkian who had been observing all this from their SkyCutter above, decide to attack the trio. Lion-O, Hachiman and Nayda use the branch of the fallen tree as a catapult and knock the SkyCutter out of the sky. The Mutant craft crashes and Monkian flees, leaving Slithe trapped in the wreck of the SkyCutter. Lion-O decides to take Slithe with them as a hostage.
Monkian reports the events that had transpired to Mumm-Ra who then summons a Ninja to finish the job which Hachiman didn't. The Ninja, in the disguise of Gumla the Tabbut, goes to the Treetop Kingdom to negotiate the release of Slithe on behalf of Mumm-Ra and the Mutants. That night, while secretly carrying out his plan, the Ninja is caught by Hachiman. The Ninja uses Nayda as a hostage, threatening to drop her from her treetop post. At that moment, the Mutants who had been waiting in the nearby bushes, attack the Warrior Maidens. As Lion-O summons the other ThunderCats, the Ninja drops Nayda but Lion-O throws the sword into a nearby tree and she grabs the sword and swings to safety. As Hachiman goes to attack the Ninja, he disappears into thin air. Suddenly the rest of the ThunderCats arrive in the [ThunderTank] and the Mutants run for their lives.
Hachiman :O