After the destruction of their home planet Thundera, the titular ThunderCats fly off to a new home, only to crash land on Third Earth after being shot down by their arch-enemies, the Mutants. Following a scuffle with the Mutants, Lion-O pursues them into the wild while the rest of the ThunderCats meet the Berbils whose village their ship unknowingly destroyed in the crash. Luckily, the Berbils are quick builders and opt to build their base while warning them of Mumm-Ra. Lion-O ends up encountering Mumm-Ra when chased by the Mutants to the villain's pyramid, with Mumm-Ra destroying the Mutants' ship before returning to his lair.