On the second day of his trials, Lion-O must race Cheetara through the dangerous lands of Third Earth. Although Cheetara is by far the fastest among the Thundercats, her speed can only last for a short time period. Lion-O takes a shorter but more dangerous route to make up for Cheetara's advantage.

The Thundercats briefly show Lion-O how the time capsule they recovered from the caveman works. Curious to see more, Lion-O activates it during his night-time watch, and gets caught in its projection rays. The rays send him back in time to Thundera, just before the final tremor that destroyed the planet. Meanwhile, the Mutants have built an evil robot monster that is destroying Cats' Lair. Back on Thundera, Lion-O encounters his father, Claudus, and saves him from a Mutant attack. In return, Claudus gives Lion-O the plans to the robot that was attacking the Thundercats in his own time.

An evil hunter lands on third earth in search of prey to hunt. His holographic machine can tell him about all of the Thundercats, including their strengths and weaknesses, and he is able to hunt down each one of them and put them in Thundrainium cells. He captures all of them but Lion-O and Snarf. Jaga warns Lion-O about Safari Joe before he sets out to save his comrades, and Lion-O is able to defeat him and release his friends.

On Lion-O's 20th birthday, he must pass a set of trials to become the true Lord of the ThunderCats. Even though he is the hereditary Lord of the ThunderCats, he must prove himself to be worthy of that title. His first test is to beat Panthro in a duel to test his strength.

Two Bolkins (sheep-like beings from Third Earth) unleash the ghost of Grune the Destroyer, a renegade Thundercat who once terrorised Third Earth and now turns his attention to the Thundercats.

Nearly all the Thundercats have been captured by the Mutants and imprisoned in Mumm-Ra's Pyramid, forcing Snarf to use his own talents to free them.

Lion-O undergoes the fourth task in his Anointment Trial.

Mumm-Ra tricks Lion-O into using the Sword of Omens against Tygra, causing the blade to break in half and lose its powers. The only way Lion-O can restore the Sword is to venture into a volcano and have the Inflamer reforge it. But the Inflamer requires a payment of gold... Has Lion-O taken on more than he can handle?

Wily Kit and Kat stand in Lion-O's way on the third day of his trials. Lion-O must beat them through the maze of infinity. Attempting to stop Lion-O and slow him down, the kids set traps in the maze. He manages however to avoid their traps. In order to beat them through the maze, Lion-O plays their own tricks on them, and tricks them into losing the race through the maze.

When a meteorite made of pure Thundrillium crashes on Hook Mountain, the Thundercats decide to use it to replenish their depleted energy supply. But Lion-O must first gain the trust of the fierce Snowman.

Mumm-Ra creates device that can make Panthro clones. He embeds the spirit of Hammerhand into his clone and sets it free to destroy Third Earth. The Third Earth inhabitants blame the Thundercats for the terror because they think its the real Panthro destroying their villages and burning their crops. The real Panthro finally encounters his clone and, with the help of Lion-O, manages to destroy it. Enraged about his defeat, Hammerhand destroys Mumm-Ra's cloning device.

An alien spaceship crash lands on third earth, and it can somehow communicate with Cheetara through her sixth sense. She becomes a part of the spacecraft, and begins to feel what it feels. The Mutants recover the space ship, and begin tampering with it. Little do they know that it is not only affecting the spaceship, but it is affecting Cheetara. In order to save Cheetara, the Thundercats set out to recover the space ship from the Mutants and fix it.

Vultureman summons Ratar-O to take him to the planet of the Snarfs to capture one of them and use it as ransom with the Thundercats. Snarf, thinking his entire race was wiped out when Thundera blew up, finds out that they have captured Snarfer, his little nephew. The Thundercats set out to find the Mutants and save Snarfer. After successfully saving Snarfer, the Thundercats are faced with the problem of returning Snarfer to his planet. Panthro cleverly invents the Feliner, a spaceship. In order for it to work, though, the machine need a hyperspace mega-condenser. Snarf reminds him that there is one on Ratar-O's Ratstar. The Thundercats once again pursue the Mutants to the wreckage site of the Ratstar to take their mega-condenser.

Mumm-ra transforms himself into a ro-bear berbil (a Thundercat ally). He acts hurt and the Thundercats let him stay at Cats Lair. One night, Mumm-ra(the berbil) puts all the Thundercats under his spell, except Snarf, because Tygra held off Mumm-ra just long enough for Snarf to escape. Lion-O was sleeping out in the woods, and Snarf ran to him and told him what happened. Lion-O came back and fought Mumm-ra, who was actually beating Lion-O. Mumm-ra was wearing special glasses so he couldn't see his reflection. Eventually, Lion-O used the Thundercat signal, broke Mumm-ra's spell and his glasses, and finally defeated him with his reflection.

With their home planet, Thundera, about to explode, the Thunderians flee to a distant galaxy. Before they can make the long journey the Mutants sweep in and destroy most of the fleet in an attempt to capture the Eye of Thundera. Due to the attack, the Thundercats find themselves on a distant planet. A grown-up Lion-O must save his Thundercats from the Mutants on Third Earth.

Mumm-Ra uses his shapeshifting abilities to pass himself off as a Thunderian so he can stage the "rescue" of Cheetara and infiltrate Cats' Lair.

With their supply of Thundrillium running low, the Thundercats set off in search of a fresh supply. Tygra detects a large deposit in a cave and goes to investigate - unfortunately, he has entered the Cave of Time and anyone who ventures in there will age rapidly until they die. His only hope is that Cheetara's speed will be fast enough to allow her to escape the Cave's effects.

Vultureman summons Ratar-O to take him to the planet of the Snarfs to capture one of them and use it as ransom with the Thundercats. Snarf, thinking his entire race was wiped out when Thundera blew up, finds out that they have captured Snarfer, his little nephew. The Thundercats set out to find the Mutants and save Snarfer. After successfully saving Snarfer, the Thundercats are faced with the problem of returning Snarfer to his planet. Panthro cleverly invents the Feliner, a spaceship. In order for it to work, though, the machine need a hyperspace mega-condenser. Snarf reminds him that there is one on Ratar-O's Ratstar. The Thundercats once again pursue the Mutants to the wreckage site of the Ratstar to take their mega-condenser.

The Driller, Mumm-Ra's servant who captured Panthro for him in Spitting Image, returns to aid Mumm-Ra once again. His job this time is to drill a hole from the Cats' Lair to the Acid lake, allowing the acid to flow in and eat away at the lair. The Thundercats set out to find the one substance that neutralizes the acid.

Wiz-Ra, Mumm-Ra's enemy from long ago, comes back every 7000 years through a portal that just happens to be in Cheetara's bedroom. Wiz-Ra wears a magical helmet that lets him control minds. When the time arrives and Wiz-Ra comes through the portal, Mumm-Ra steals the helmet. Cheetara helps Wiz-Ra get the helmet back and free the ThunderCats from its spell.

Wilykit and Wilykat try to stop a band of living gargoyles from robbing a Wollo but fail to do so and trail the gargoyles to a nearby Tower. Wilykat is trying force the door open when he suddenly disappears, leaving Wilykit - who was looking for another way in - close to panic. She and Lion-O (alerted by the Sword of Omens) must now brave the Tower's potentially lethal traps in order to rescue Wilykat.

While investigating an earthquake, Tygra stumbles across the Garden of Delights and meets a talking plant named Silky, who offers him her fruit. But it turns out that "Silky" is another of Mumm-Ra's disguises and eating the fruit seems to have affected Tygra's mind.

Mumm-Ra and the Mutants plot to open the Time Warp Prison and release the evil Princess Ta-She in order to use her deadly Doomgaze against the Thundercats.

The Mutants, once again searching for a way to destroy the Thundercats, recover some more items from their spaceship. They take their ship's engine and convert it into a giant fireball-shooting machine. The mutants terrorize Third Earth with their fireballs, until the Thundercats go to castle Plunn-Darr and destroy their dangerous new weapon.

Wilykit and Kat accidentally free an ancient monster that was banished to an old shrine centuries ago. The evil monster, Mongor, begins to destroy the planet, and is much stronger than the Thundercats. He captures all the elder Thundercats with his powers. Wilykit and Kat return to the shrine to find a way to destroy him. They report back to Lion-O and tell him that in order to destroy Mongor, he has to fight the monster without looking at it. Lion-O defeats the monster and all is returned to normal.