The inhabitants of the planet Thundera evacuate just before it is destroyed. They were pursued by a band of mutants. All but one of their escape ships was destroyed. Only a small group of Thunderans (Thundercats) remained. With only half engine power, the group, which was led by Jaga, had to set a course for the nearest planet. Jaga commanded their ship while the other 7 were in their stasis tubes. Jaga died on their journey to Third Earth and their ship crashed there. Soon they made friends with various groups in the area and they designed a fortress. Mumm-Ra the centuries-old embodiment of evil, along with the mutants that destroyed the rest of the Thunderans are a constant threat. But Lion-O, the new leader of the Thundercats, with his weapon the Sword of Omens, leads the surviving Thunderans and their new friends on a never-ending crusade again Mumm-Ra and the evil mutants. The task is made more difficult by the fact that, at will, the undead Mumm-Ra can focus external energies and because Mumm-Ra, the Ever-Living, to terrorize and plague the ThunderCats. But they always manage to beat him back down, and Mumm-Ra reverts, weakened and slinking back to his Pyramid.
Intro 2 originally preceded the opening titles in the "movie" version of the origin story. It appeared on the original fhe video release as well. The movie, named "Exodus" after the premiere episode, was a truncated version of the first 5 episodes, cutting quite a lot of material out. So the movie still had the same intro, but "Intro 2" was shown right before it.
Thanks to sm2000 for the intro 2 information.