The Ancient spirits of evil show Mumm-Ra the story of King Arthur, and explain to him that the strongest sword that ever existed was Excalibur, Arthur's weapon. Mumm-Ra disguises himself as Arthur, and returns to lake where the sword is guarded by the lady of the lake. Fooled by his disguise, the lady gives Mumm-Ra Excalibur. The disguised Mumm-Ra challenges Lion-O to a dual, and Lion-O foolishly accepts the challenge. The sword clash, and Excalibur is victorious. The eye of Thundera is split in two, and the Thundercats become powerless. Merlin, Arthur's teacher and a famous magician stops Mumm-Ra from destroying the Thundercats, and restores the eye to its original state. (thetvdb.com)