Lion-O frees the Thundercubs from Mumm-Ra's spell and the Thundercubs safely leave New Thundera. Lion-O and Snarf must confront Mumm-Ra and get the Book of Omens. After they defeat Mumm-Ra, they ask Mu-Mutt (using a magical flute) to use Mumm-Ra's ""device"" to get them back to Third Earth. Meanwhile the Lunataks capture some Berbils and the Thundercubs return to Third Earth and they and the Thunderkittens must go to Sky Tomb to free the other Thundercats and the Berbils. After everyone is freed, they are all cornered by the Lunataks, but Lion-O and Snarf arrive to save the day. The Thundercubs must then go in the Cave of Time to become Thundercats again and Mumm-Ra is abandoned on New Thundera and the Ancient Spirits bring him back to Third Earth.