Wilykit and Wilykat are playing in the forest when they discover that the Mutants have enslaved a race of primitive beings whom they refer to as Brute-Men and are forcing them to build Castle Plun-Darr. The Mutants are extremely cruel in their treatment of the Brute-Men and beat them severely if they falter. Before the Thunderkittens can do anything, they are spotted by Monkian who chases them but they manage to give him the slip.
Meanwhile, construction of the Cats Lair is proceeding very well thanks to the efforts of the Berbils. Wilykit and Wilykat arrive at the lair and report the Mutants' actions to the rest of ThunderCats. Ro-Bear-Bill, who is also present there, further informs the Cats that the Brute-Men are gentle beings who lack much intelligence. The ThunderCats, led by Lion-O, set-off to rescue the Brute-Men in the ThunderTank, leaving Kit and Kat to guard the Lair.
The ThunderCats arrive at Castle Plun-Darr and the ThunderTank breaks through the surrounding barricade with ease. Lion-O enters the Castle while the rest of the Cats split up and go round the sides and back of the Mutants' fortress. Cheetara encounters Jackalman, Tygra runs into Monkian and Panthro squares off with Slithe. All three Mutants are easily overpowered by the Cats.
In the mean time, Lion-O manages to locate the Brute-Men's cells and sets them free. Slithe arrives and after witnessing the freed Brute-Men, he decides to turn the situation to his advantage. He throws a canister of Warp Gas in the midst of the Brute-Men and the vapors cause the meek creatures to turn into raving monsters and they start attacking Lion-O. They then turn their attention to Panthro who has just arrived inside the Castle but just as they are about to attack Panthro, the effects of the Warp Gas wear off and the Brute-Men revert to their docile nature. Slithe throws another canister of the gas but Lion-O uses the Sword of Omens' force field to push the gas back towards the Mutants who now end up being on the receiving end of the Brute-Men's rage and quickly retreat inside the castle. The Brute-Men rejoice at being freed and return to their homes.