Lion-O is happily playing with Snarf in their "Frolicking Field" and doesn't have a care in the world, unfortunately he's accidentally stomping all over the miniature village of the Micrits. The tiny people want to defeat the "giant orange monster" but such a quest would require a brave hero. Luckily, the Micrits have such a hero: Prince Starling. Now, let us the tell you the epic tale of Prince Starling as he journeys to find the fountain of growth and defeat the wicked Lion-O, who has no idea any of this is going on.
The Thundercats are exhausted from fighting bad guys all the time so Lion-O suggests they spend a relaxing summer fun day at the beach, with no fighting. Unfortunately the beach is home to the Crabmen, giant, clawed humanoids that fight anyone who steps on their sand. Looks like summer fun day might involve some fighting.
Lion-O never pays attention during Tygra's training sessions so when he's asked to do a pop quiz on "sword magic" he accidentally opens a portal to the Astral Plane and releases the Nether Witch. The Witch imprisons Lion-O and Tygra in her nightmare realm and it's gonna take a lot of studying to figure out a way home. Wonder if the Astral Plane has any great teachers lying around.
Lion-O is tasked with mapping the beaches of Third Earth, which leads to him somehow draining the oceans of all water entirely. Woops. But a mysterious scientist with a robotic frog says they can fix everything by traveling under the earth, which is just the kinda crazy adventure Lion-O is all about.
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?