Fleeing the destruction of their home planet Thundera, Lion-O and the ThunderCats land on the mysterious, dangerous, all-around awesome world of Third Earth! But their enemies, the Mutants of Plun-Darr have followed them and it turns out Third Earth already has a Mumm-Ra problem - can Lion-O learn the ropes of being a leader in time?

Lion-O wants to prove to the ThunderKittens that he's not just some boring old adult like Tygra, even if it means ignoring his instincts and messing with an obviously evil crystal.