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 the 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 swords 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