As their home world is about to explode, the Thundercat Nobles escape. Their engine is damaged so they can't go to their intended destination. While the others enter sleep chambers to survive the long journey, their leader Jaga remains awake to pilot the ship as long as possible since their navigation system is damaged. One they land on Third Earth, the Thundercats must deal with old enemies who have followed them, and face all the challenges that await them on Third Earth.
The Thundercats encounter the Berbils, when the Berbils are attacked, the Thundercats defend the Berbils. The Berbils explain that the group who attacked them were ordered to so by some Giantsour. The Thundercats then have to protect the Berbils from the Giantours.
While investigating an earthquake, Tigra stumbles upon a magical garden and meets a talking flower named Silky who offers him her fruit. It turns out that she is another one of Mumm-Ra's disguises and the fruit has made Tigra his slave.
A tomb is accidentally opened, releasing a ghost who used to terrorize Third Earth. The ghost has a special interest in the Thundercats. The Thundercats must figure out who the ghost was in life and his connection to their past.
Lion-O accidentally activates the time capsule, which hurls him back in time to Thundera exactly one day before its destruction. Meanwhile, the Mutants assault Cat's Lair with their most powerful and deadly weapon yet.
Lion-O is the hereditary Lord of the Thundercats. Now he has reached the age where he must prove his worthiness. He must face each of Thundercats in different trials/tests, without the Sword of Omens. First up, Panthro.
Lion-O's trial to become the Thundercat Lord continues. Now he must race against Cheetara. But now the Mutants and Mumm-ra have found out about his trials. They'll stop at nothing to keep Lion-O from succeeding.
Lion-O must now face off against Wily Kat and Wily Kit. It's a race through a cavern, with Wily Kat and Kit, playing tricks on Lion-O. They must also contend with Mutants, and Lion-O meets a race who dwell in the caverns and who don't like people from the outside.
Lion-O's faces off against his final Thundercat, Tygra. Tygra can create illusions, he's been conserving his energies for these trials. Can Lion-O tell which danger is real, and which is not? The mutant continue their interference.
Mumma-ra learns about Excalibur, the greatest sword ever created. He changes into King Arthur, and asks the Lady of the Lake for Excalibur. He meets with the Thundercats, who greet him as a potential ally. Soon "Arthur" challenges Lion-O to a duel, which he foolishly accepts. They will all learn which sword is stronger, the Sword of Omens, or Excalibur.