Having survived their crash on an alien planet, all of the Thundercats get to work, taking stock of their supplies and making plans to build their new home... well, almost all. The Lord of the Thundercats seems more interested in exploring and having fun than in planning for their future, causing his team to realize just how awkward it is being led by a kid trapped in a grown man's body. This is going to take some getting used to...
Meanwhile, three of the Mutants are scouting the planet and come across some familiar ruins. Everything looks deserted, but they hear a voice calling out to them. They soon find themselves face to face with a demonic mummy who calls himself Mumm-Ra. He is thrilled that they have landed here because "As long as evil exists, MUMM-RA LIVES!". He thanks them by burying their ship in quicksand, thus destroying their only means of leaving the planet and communicating with the rest of their army. Looks like they have no choice but to serve him now! They shouldn't mind, since they all want the same thing — the Eye of Thundera. If they destroy its current owners, the Thundercats, for him, Mumm-Ra will, well, let them live.
At least Mumm-Ra's kind enough to inform us that the planet the heroes and villains have landed on is called "Third Earth."
This episode provides examples of:
- After the End: It's established here that the Thundercats and the Mutants have landed on a future Earth after some global catastrophe that wiped out human civilization, leaving behind mostly ruins.Monkian: So there was some kind of civilization on this ridiculous pebble of a planet.
- Curb Stomp Cushion: Mumm-Ra quickly disarms Lion-O of his sword. Lion-O can't land a punch on him, and when he hits Mumm-Ra with a tree, he deals no damage. He does manage to flip Mumm-Ra over his head. Mumm-Ra ultimately overpowers him and is about to kill him before encountering his Weaksauce Weakness, his own reflection.
- Eiffel Tower Effect: The "ruins" spotted by the Mutants place them and the Thundercats in what used to be Egypt.
- From Bad to Worse: As if the Mutants weren't bad enough, now the heroes have the more evil and more powerful Mumm-Ra to deal with!
- Hypocrite: In a sympathetic way — Lion-O insists "I'm not a kid anymore!", yet all he wants to do is play around instead of share the adults' responsibilities. Understandable given his unique circumstances, but still something he has to learn to get over sooner rather than later.
- The Leader: With Lion-O busy goofing off like the kid he really is, Tygra is the one in charge, giving out orders and assignments and helping everyone keep things in perspective. (It will be mentioned a few episodes down the line that Tygra is the leader of the Thundercats, presumably until the heir was ready to become Lord of the Thundercats several years in the future...)
- Magi Babble: Rule #1 about the Sword of Omens — it will not let you use it to harm the innocent. It prevents Lion-O from killing animals for food.
- Mood-Swinger: The episode opens with Panthro throwing a tantrum over how much of their equipment was damaged in the crash. Then some switch gets flipped and he immediately relaxes, planning to use some parts to build a Thundertank.
- Non-Indicative Title: "Unholy Alliance" implies the episode would be about a mutual Villain Team-Up. Instead, Mumm-Ra essentially enslaves the Mutants into helping him. They didn't sign up as his mooks willingly. Whatever you call their relationship, it's not an "alliance."
- Overnight Age-Up: Portrayed realistically. Just because he aged while asleep doesn't mean Lion-O grew up, as the top-page quote explained. He still has the mind of a little boy, even playing around with the Sword of Omens like it's a toy! It eventually starts to sink in for him that being grown up doesn't mean getting to do whatever he wants but that he has a ton of new responsibilities he now has to learn how to handle. It might not be fair, but that doesn't mean it's not true.
- Reality Is Unrealistic: As Life After People demonstrates, ancient stone monuments like the pyramids will last longer than modern construction like skyscrapers and bridges. If any aliens who visit a post-Apocalyptic Earth do find anything humans left behind, the pyramids and the Sphinx are great candidates.
- Second Episode Introduction: We're introduced to the Big Bad of the series, Mumm-Ra.
- Supernatural Aid: Lion-O now adds the Claw Shield to his arsenal.
- Transformation Is a Free Action: Averted — the Mutants fight to stop Lion-O from completing the chant to call the Thundercats.
- The Unreveal: What happened to destroy Earth's civilization? We never find out.
- On a smaller scale, both Lion-O and Panthro are worried when Cheetara and Tygra just... don't respond to the Thundercats' signal. It is never revealed what they were doing together... alone... in a dark cave... that had them so distracted that they completely missed the Eye of Thundera's call...!
- What the Hell, Hero?: Jaga appears to give Lion-O a lecture on his childish behavior.
