Lion-O is alarmed by the Sword of Omens' warning and using "Sight beyond sight" he sees that Jaga has been imprisoned in the Astral World by Nemex, a three eyed, four armed creature, who plans on keeping Jaga there for all eternity. Lion-O quickly informs the other ThunderCats about this and they reach the conclusion that only the Nether Witch might be able to help them. Mumm-Ra, who had been watching the ThunderCats from his cauldron is very pleased as he and the Nether Witch are one and the same. He then transforms into the Nether Witch.
The ThunderCats drive out in the ThunderTank to the Nether Witch. Lion-O insists that he must embark on this mission alone so the rest of the ThunderCats drop him off at the Bridge of Slime and leave. As he crosses the bridge, the Gaw Rak Rak, a two headed monster, emerges from the water and lunges at him but Lion-O manages to subdue it and crosses the bridge to reach the Pit of the Nether Witch.
Meanwhile inside Castle Plun-Darr, Mumm-Ra has informed the Mutants about Lion-O's absence and that they must use this opportunity to their advantage. Vultureman prepares a "Thundrainium Cannon" and the Mutants set out in their vehicles to attack Cats Lair.
Back inside the Pit of the Nether Witch, Lion-O encounters a number of strange apparitions and creatures before finally coming face to face with the Nether Witch herself. Lion-O asks her to transport him to the Astral World but she warns him that once he is there, he can never return back to Third Earth. Willing to do anything to save Jaga, Lion-O agrees and the sorceress starts her incantation. As Lion-O starts de-materializing, Mumm-Ra reveals his true self and the young lord realizes that he has been tricked just before he disappears completely.
Lion-O materializes in the Astral World, near the fortress where Jaga is being held prisoner. Using the line from his Claw Shield, Lion-O begins ascending the steep walls of the tower. Suddenly, he is attacked by the Astral Moat Monster which jumps up at him from the moat surrounding the tower. The creature then starts pursuing Lion-O by using its sharp claws to to climb the wall. Lion-O zaps it with a beam from the Sword of Omens and the creature falls back into the water, never to be seen again.
Once inside Jaga's prison cell, Lion-O is confronted by Jaga's captor, Nemex who shoots energy beams at the young lord. Lion-O counteracts his beams with the Sword of Omes and knocks down Nemex. Lion-O then quickly frees Jaga and as the two are running through the prison, they hear calls of help from a nearby cell. They break into it to find, Brodo, an old man who was once a powerful magician on Third Earth but was tricked by Mumm-Ra and imprisoned in the Astral World. Lion-O and Jaga take Brodo with them as they escape the prison fortress.
Outside the prison, the Sword of Omens shows Lion-O that Cats Lair is being attacked by the Mutants and the Thundrainium shells that they are using in their cannon is weakening the ThunderCats, making them unable to fight back. Lion-O feels helpless as he cannot help his friends when they need him the most. At that point, Brodo reveals that he will use his powers of good to transport Lion-O to to his own world and he begins his incantation.
Lion-O materializes back outside Cats Lair and using his sword, destroys the Thundrainium Cannon. The arrival of their lord gives the rest of the ThunderCats renewed strength and they are able to defeat the Mutants