I. The birth of the swords
The story begins with Mumm-Ra and his ship the Black Pyramid. Mumm-Ra wanted to control the whole universe and for that purpose he searched for magical stones across the stars. The Ancient Spirits of Evil helped him in his quest. These spirits were his masters (maybe teachers).
In his quest various species helped him, but most of them were slaves. The only specie that served him willingly were the cats. He had chosen a commander named Leo to help him conquer the universe. Leo and his cats were loyal to Mumm-Ra, but as timed passed more of them felt that they following the wrong leader. Mumm-Ra wiped out civilizations just to get hold of the magic stones. But Leo shared Mumm-Ra’s vision to create a united universe under Mumm-Ra’s command.
Mumm-Ra had an artifact called the Book of Omens, which helped him find the stones. It was created with technology and magic. But before the book could lead Mumm-Ra to the fourth and final stone the Ancient Spirits spoke to Mumm-Ra and ordered him to create the ultimate weapon. They told him to use the star of Plun-Darr to create the swords material.
Mumm-Ra did it without hesitation destroying an entire galaxy in the process. He harvested the metal with a satellite. Then the Ancient Spirits possessed a cat blacksmith and made the Sword of Plun-Darr and a gauntlet.
However when Leo saw the destruction of the Plun-Darr galaxy he finally decided to turn on his master. After the creation of the sword a lot of scrap metal remained. Leo collected them and brought it to the same blacksmith who created a much smaller sword called the Sword of Omens. The smith also created a gauntlet.II. Rebellion
Mumm-Ra traveled to the planet, which hold the fourth stone called the War Stone. He intended to put the stone in his sword. He made the mistake to sent Leo to fetch the stone from the returning forces who had taken it from the planet. Leo put the War Stone into the Sword of Omens and turned openly against Mumm-Ra. In the process he freed all the enslave animals.
At the end he defeated Mumm-Ra who lost most of his powers. He tires to escape and close himself in a small coffin like pod (maybe an escape pod?). The cats couldn't open it, so they decided to break the controls and trap Mumm-Ra in it.
However during the epic fight between Leo and Mumm-Ra the Black Pyramid is severely damaged and crash on a planet called Third Earth.
The crew survives the landing. Leo became the first Lord of the Thundercats. They separate the stones to hide from Mumm-Ra in case he would return. They also hid the Sword of Plun-Darr and Mumm-Ra’s gauntlet.III. Rise of Thundera
The Thundercats became the dominant species of Third Earth. They took all the good lands, and they didn't care what is outside their borders. Somehow technology became a myth just like the stories about Mumm-Ra. The Thundercats let other species into Thundera but they are only second class citizens.
The Lizards attack Thundera from time to time, to try and get some lands for themselves. During this time a new order helps the Lord of the Thundercats called the clerics. They are persons with extraordinary capabilities. Some of them even could use magic. The clerics hid the Book of Omens in a tower for safekeeping.IV. The fall of Thundera
Many centuries after their arrival, the king Claudus (descendent of Leo) have found the myth of the Book of Omens. He feels he needs the book's wisdom, so he send his two finest generals Panthro and Grune to find the book.
They travel for a long time but they didn't find it. Then one night Grune hear a calling voice in the desert. He is the only one who hears it. The voice calling him, and promising him the power he had been wanted. Grune follows the voice and Panthro follows him. They reach the Black Pyramid and they enter it. Inside Grune frees Mumm-Ra and defeat Panthro. He sworn loyalty to Mumm-Ra.
Mumm-Ra convince the lizards to join him. He builds a technologically advanced army and prepares to destroy Thundera.
Grune returns to Thundera alone with a big rock with a lot of gold in it. It is a present for the king. He informs Claudus that Panthro is dead and they haven't found the book. Despite of that the king very happy to see at least one of his general had returned so he makes a big celebration.
Later in that night, lizards crawl out of the big rock. It was a Trojan horse all along. To create diversion the lizards attack Thundera. They don't use their superior firepower in the first attack to deceive the cats. The Thundercats managed to beat the lizard army. But in the meantime the small unit in the city places beacons in key locations.
When they are ready the real attack begins. Guided missiles hit the beacons locations destroying the city. The lizard army returns with laser weapons and giant mechs. The Thundercats medieval technology simply not a match for them.
During the fight Mumm-Ra kills Claudus in front of the eye of his sons Lion-O and Tygra. Both princes captured. The King's wizard and the head of the clerics Jaga also captured. But the two princes manages to escape and find Mumm-Ra, who is torturing the old wizard to remove a protective spell from the Sword of Omens so he can have it.
The two princes helps Jaga to escape. Lion-O takes the Sword of Omens and the gauntlet. During their escape they stop for a short ritual and Lion-O became the next Lord of the Thundercats. Then Jaga stay behind to buy them time to escape.
The next morning Lion-O burns the body of his father and left the destroyed city.