The next day Thundera is attacked by Lizards who have acquired salvaged technology. It is soon discovered that Grune has joined the Lizards and that Panthro is being held hostage. Lion-O comes to his father's aid, allowing Claudus to rescue Panthro. But Panthro murders the king and reveals himself to be Mumm-Ra , an ancient nemesis of the Thundercats, in disguise.
Lion-O and Tygra are captured along with Jaga and Cheetara. However, due to his kindess to the Lizard prisoners, Lion-O was given a key hidden in his meal, allowing him to escape with Tygra to mount a rescue. Mumm-Ra wants the stone in the sword which he claims is his, and tortures Jaga to get him to remove a curse that prevents evil from touching the Sword of Omens. Lion-O and Tygra rescue Jaga and Cheetara, who is now revealed to be one of Jaga's clerics, with Lion-O grabbing the sword to blast Mumm-Ra out into open as the rising sun's rays harm him.
Forced to leave Jaga, who sacrifices himself to ensure their escape, Lion-O, Tygra, and Cheetara undertake the quest to find the Book of Omens.
- Mumm-Ra uses his shape-shifting ability for the first time on screen.
- Thundercats.ws :Reminder – Thundercats Airs Tonight "Tonight’s premiere gives us the first two episodes of the show; The Sword of Omens and Ancient Spirits of Evil." (Archived on Internet Archive Wayback Machine on May 06, 2020)