After overhearing Snarf telling the Thunderkittens and Pumyra a story about Mumm-Rana's magic belt and Queen Luna (Luna's grandmother), Luna and Amuk journey to the white pyramid and take Mumm-Rana's belt, destroying her pyramid in the process. Pumyra tries to stop her but she gets sealed up in the pyramid. Amuk becomes jealous when Luna no longer needs him and during his ""little temper"", he accidently frees Pumyra. Then Luna forces Slithe and Monkian to help her ambush Lion-O and Panthro. During the battle with ""Princess"" Luna, Lion-O uses the sword to restore Mumm-Rana and she retakes the belt and defeats Luna. Amuk later arrives and destroys the belt.

Back visiting their old wreckage site, Lion-O and Snarf stumble upon the Thunder-scope, a legendary thunderian telescope that locates lost thunderians, and their belongings. Mumm-Ra, interested in the Thunder-scope so he could find the treasure of thundera, ambushes Lion-O. He actually kills him in this scene. Before Lion-O dies, he manages to take out a lens from the Thunder-scope so Mumm-Ra could not completely use it. Jaga appears to Snarf, and Lion-O's dead body, and revives him. Mumm-Ra comes back for the missing piece, and Lion-O has to stop him.

Mumm-Ra summons Malcar, an ancient alchemist, to return from the dead. The alchemist returns old and weak, as he was when he died. Mumm-Ra takes him to the forest of youth to restore his youth and power. Malcar regains his powers, and uses them against the Thundercats. He transforms all the Thunderillium on the planet into Thundrainium.