Lion-O is haunted by the recurring nightmares of Thunderians being killed on Thundera while he fled to the safety of Third Earth with Jaga and the other ThunderCats. Jaga later tells Lion-O that 3 more Thundercats have, in fact, survived and are also living on Third Earth. The ThunderCats decide to search for the 3 new Thunderians, but Mumm-Ra has plans as well and takes the new Thunderians as his prisoners to Fire Rock Mountain. Lion-O loses the Sword of Omens in an avalanche while Panthro battles the Fist-Pounder with the ThunderTank as the ThunderCats close in on rescuing the Thunderians from Fire Rock Mountain, but they'll need supernatural help if they are to pass through the Thundranium gasses to reach them.
