The Unholy Alliance is the second episode of the original series. It originally aired on 23 January 1985.
The ThunderCats are busy trying to salvage as much equipment and food rations as they can from the wreck of their fallen spaceship. Panthro, while initially angry that most of their equipment has been destroyed, is confident that enough Thundrillium is left to build a ThunderTank. Lion-O decides to scout the surrounding area, taking the Sword of Omens and Claw Shield with him while Tygra and Cheetara go searching for a suitable location to build Cats Lair. When Panthro and Cheetara question the wisdom of allowing Lion-O to proceed on his own, Tygra reminds them, "Remember, Lord Lion-O spent ten galacto-years in a suspension capsule. During that time, he grew in size, but he did not grow up. He missed those first-hand life experiences through which one matures."
Meanwhile, the Mutants, while searching for the ThunderCats as well as a suitable location to build Castle Plun-Darr, come across an old pyramid. As they approach the pyramid, it starts to emit lightning, forcing the Mutants to land their ship. Inside the pyramid, Mumm-Ra emerges and offers the Mutants a chance to ally with him to obtain the Eye of Thundera. At first the Mutants reject his offer and threaten to blow up Mumm-Ra's pyramid. Mumm-Ra in turn retaliates by sinking their spaceship where it stands. The Mutants, now weaponless without their ship, have no choice but to join forces with him. He teleports them to the woods where Lion-O is scouting alone.
The Mutants re-materialize in the woods and ambush Lion-O but he manages to call for help using the Sword of Omens and Panthro, Wilykit and Wilykat arrive to rescue him. Defeated by the ThunderCats, the Mutants teleport back to Mumm-Ra's pyramid. Lion-O, surprised as to why Tygra and Cheetara did not see the Cat signal, goes off in search of them.
Back inside the Black Pyramid, Mumm-Ra ridicules the Mutants for failing miserably to obtain the Sword from Lion-O. The Mutants retort by taunting Mumm-Ra which greatly angers him and he decides to show them his true power by transforming into his Ever-living form and flying out of his pyramid.
While searching for Tygra and Cheetara, Lion-O comes across a strange black lake which is filled with thick, hot, bubbling black ooze. Mumm-Ra suddenly appears and a fight ensues between him and the young lord. The Sword is knocked from Lion-O's hands and falls into the black lake. Lion-O retrieves it and tries to call the other ThunderCats, but the Eye of Thundera, now covered by the black ooze, does not respond. Mumm-Ra manages to pin down Lion-O and in a desperate attempt to shield himself from the fiend, Lion-O holds up his Claw Shield in defense. Mumm-Ra, upon seeing his reflection in the Claw Shield, is horrified and driven back to his pyramid.
- Mumm-Ra (1st appearance)
- Cheetara's left hand, and maybe part of her left leg is missing when she stands with the rest of the ThunderCats towards the beginning of the episode.
- Towards the end of the episode, Snarf's right ear disappears for a few seconds and then reappears.
- It is never explained why Tygra and Cheetara were not summoned when the Eye of Thundera called.
- In the previous episode, "Exodus," the adult Lion-O's outfit is torn and tattered due to his having grown in size. However, in this episode and all others afterwards, his outfit is intact.
- It is shown for the first time that Mumm-Ra is afraid of his own reflection, and can cause him to be driven away. Lion-O and the other ThunderCats would go on to use this weakness to defeat Mumm-Ra in a number of later episodes such as Snarf Takes up the Challenge, The Mumm-Ra Berbil, Trapped, The Transfer and Monkian's Bargain among others.
- The stock footage of Mumm-Ra transforming from his mummy form to Mumm-Ra the Ever-Living is used for the first time in this episode and would be re-used regularly throughout the series.
- The Ball of Light that Mumm-Ra uses to guide the Mutants into his pyramid would be used by him again in the episode The Demolisher
- "The Unholy Alliance" was initially broadcast as part of a feature-length pilot, featuring this episode and Exodus, cut together to create a one-hour special. When released on home video as Exodus: The Movie, the episodes "Exodus" and "The Unholy Alliance" were joined by edited versions of the episodes Berbils and The Slaves of Castle Plun-darr, all cut together to create a "movie" version of the first four episodes.
- Lion-O: It was its own reflection that drove it off. Yet it feared nothing.
- Tygra: Except the evil in its own unspeakable face.
- Mumm-Ra: Have no fear.
- Slithe: I am Slithe, and I fear nothing!
- Monkian: Look - those statues.
- Jackalman: What do you think? An ancient race of Mutants?
- Slithe: No wonder I like this place.