Lion-O competes against Tygra in a ritual that will choose who the next King of Thundera will be.
Meanwhile, the great warrior Grune returns.
The lizards attack Thundera with unknown technology while King Claudius is apparently murdered by an old friend.
Meanwhile Jaga is captured and is tortured to reveal the location of the Book of Omens. After Thundera falls, Lion-O, Cheetara & Tygra begin the search for the book.
The cats begin their journey on the edge of the Sand Sea where they are captured by the Fishmen whose captain in on a quest to destroy a giant sand beast.
Lion-O joins the captain in his quest while the other cats see Lion-O going down a path of blind vengeance instead of searching for the Book of Omens.
While Slithe and the lizard army hunt the Thundercats, they take refuge in a huge brier patch where the meet the Petalars, a race of flowering beings that despite living a short time, teach Lion-O a thing about life.
In order to repair the Thundertank, the cats must search an abandoned mine in search of Thundrillium. While searching, Panthro goes missing.
Using the Sword of Omens, Lion-O sees Mumm-Ra has forced the location of the Tower of Omens from Jaga which contains the book.
The cats double their effort to find the book before Mumm-Ra.
After recovering the Book of Omens, Lion-O uses the Sword of Omens to power it which results in his getting pulled into its pages where he meets Jaga's spirit.
Jaga shows Lion-O how history unfolded through the eyes of his ancestor Leo.
After Lion-O looses the Sword of Omens to an expert swordsman, he challenges him to a rematch but first must find a master sword maker to make him another blade that can match the expert's abilities.
The Thundercats are contacted by the Berbils, a race or robotic bears, in order to help them against a slave trader named Conquedo.
When the Book of Omens detects the first of three magical stones, it leads them to a meditation temple inhabited by a group of elephants.
In order to find the stones, the elephants insist Lion-O become one with the Sword of Omens.
While searching the Forest of Magi Oar, the Thundercats come across a paper factory that is routinely being attacked by a fire breathing bird. However things aren't exactly what they seem to be when the cats discover who the true aggressor is.
The Thundercats return to the Elephant's temple when Lion-O believes he has become one with the Sword of Omens.
When Lion-O & Tygra search for the Spirit Stone on the astral plan, old memories of aggression, insecurity & jealousy hamper their search.
Meanwhile Panthro has a final confrontation with Grune.
In order to restore his failing numbers, Mumm-Ra instructs Slithe to recruit Kaynar and Addicus as his new generals.
Meanwhile, after discovering Cheetara's true feelings for Tygra, Lion-O's leadership abilities are effected by jealousy.
After Lion-O is killed in a sneak attack by Slithe & the lizards, he is given a second chance at life by Jaga & the Eye of Thundera, but he must first win a series of tests.
Meanwhile the other Thundercats must deal with the loss of Lion-O and escape the Lizards.
Lion-O has returned from the dead but only until sunrise. He must return to Mumm-Ra's pyramid to save the other cats.
While looking for a shortcut through the mountains, Lion-O & Tygra find a long lost Tiger clan and even more surprising, Tygra's father is among them.
While seeking answers from his father, Tygra uncovers more than he bargained for.
With Tygra & Lion-O away, Cheetara & Panthro decide to teach Wilykit & Wilykat how to survive in the wild.
The lessons ultimately remind Wilykit & Wilykat about their times with their parents.
While looking for supplies in Dog City, the cats run across a flyer featuring fellow cat, Pumyra.
Lion-O decides to liberate her when it's learned she is a slave who fights for the amusement of spectators.
After Pumyra tells Lion-O cat slaves are toiling in the mines looking for the Sword of Plun-Darr, he, Cheetara, Tygra & Panthro split up to save the cats and uncover the sword before Ratar-O finds it and uses it to rule.
After learning the Sword of Plun-Darr has been found, Mumm-Ra goes after the Thundercats in order to gain it's evil powers.
The Thundercats require the assistance of a snake oil salesman named Ponzie to defeat Mumm-ra's latest monstrous form.
Vultaire turns from enemy to ally, freeing the Cats and joining Tygra in an aerial counter-attack on Mumm-Ra. A battle for the Tech Stone ensues.
Source: Radio Times