Four Seasons of Thundercats
Season 1
ThunderCats follows the adventures of a noble team of heroes, cat-like humanoid aliens from the planet of Thundera. After fleeing their home planet, the Thundercats arrive on a remote planet dubbed "Third Earth." Together, the ThunderCats and the friendly natives of Third Earth construct the "Cat's Lair," their new home and headquarters, but before long, the Mutants have tracked them down. The intrusion of these two alien races upon the world does not go unnoticed, however - the demonic, mummified sorcerer, Mumm-Ra, recruits the Mutants to aid him in his campaign to acquire the Eye of Thundera and destroy the ThunderCats so that his evil might continue to hold sway over Third Earth.
Thundercats Ho!
The 1986 TV movie "ThunderCats - Ho!" featured the first major shake-up to the status quo of the series, introducing three new ThunderCats who had also survived the destruction of Thundera. A massive cast of returning heroes and villains were incorporated into the story to make it the most epic ThunderCats adventure yet produced, and it concluded with the apparent destruction of Mumm-Ra.
Season 2
When the series returned in 1987, however, this was quickly proven not to be the case in the opening mini-series, "Mumm-Ra Lives!", as the evil wizard survived his fate. "Mumm-Ra Lives!" set the pattern for the show's final three seasons, which each began with a five-part mini-series that established the new characters and concepts that would go on to influence the rest of the season. In the case of "Mumm-Ra Lives!", these concepts included the debut of the villainous Lunataks, who became a third faction that existed for the rest of the series, and the new team of ThunderCats from "ThunderCats - Ho!" being given their own headquarters, vehicles and so forth.
Season 3
The 1988 season began with "ThunderCubs," a miniseries named for its plot about the ThunderCats being transformed into children, but which was principally about Mumm-Ra reconstructing Thundera in order to retrieve both the weapon that had originally destroyed it, the Sword of Plun-Darr, and the legendary Treasure of Thundera. In the course of the adventure, the treasure - containing the Book of Omens, a tome holding all the secrets of the ThunderCats, and many other mystical items - was scattered across the New Thundera, ushering in a new concept for the series: a season with an actual story arc. Continuity between episodes became tighter as the ThunderCats, Mutants, Lunataks and Mumm-Ra alternated their adventures between Third Earth and New Thundera, searching for the treasure and exploiting its powers. The season also featured the running theme of the Ancient Spirits of Evil having to take a more active hand in pushing Mumm-Ra into action, culminating in another unique feature of the season - an actual finale episode, "The Last Day," in which the spirits give Mumm-Ra one last chance to destroy the ThunderCats. Ultimately, the villain failed, and was banished to the farthest corner of the universe by the spirits.
Season 4
However, Mumm-Ra returned for the 1989 season. In the opening miniseries, "Return to Thundera!", the ThunderCats returned to New Thundera to rebuild their society, but before departing, they destroyed Mumm-Ra's pyramid, enraging the Ancient Spirits of Evil to the point that they brought Mumm-Ra back, and installed him within a new pyramid on New Thundera. The season proved to be quite divorced from what had gone before, with adventures consigned almost entirely to the wild and wonderful locales of New Thundera, and most villainous opposition coming from either Mumm-Ra or assorted new villains. The Mutants, Lunataks and Captain Cracker all returned for one episode each, however. In the series finale, several conclusions are reached: Mumm-Ra stands up to and asserts himself over the Ancient Spirits of Evil, the mystery of the Book of Omens was at last solved, and the tumultuous and terrifying environment of Thundera was at last rendered peaceful and pristine.