Inside the Foot Genetics lab, the Turtles learn of past experiments that turned human subjects into monsters. The brothers soon encounter a group of these creatures who are at first hostile, but eventually become allies. The Turtles now find themselves helping these creatures evade the attacks of similar, but mindless monsters. This new partnership takes the Turtles deeper into the seemingly haunted catacombs, where creatures, and later Donatello, have disappeared.
This episode provides examples of:
- The Berserker: The other monsters, among which there are counterparts to Quarry, Stonebiter and Razorfist.
- Big Creepy-Crawlies: The fourth of the mindless monsters is a spider-like creature.
- Casual Danger Dialogue: Raph briefly calls Mike an oddball, while defending him from one of the mutant monsters. Mike stops fighting and spends some time, offended and complaining about being called "odd". When Don tells him that he is just being sensitive, he gets even more offended.Raph: Watch your back you oddball!
Mike: You just called me odd didn't you? [Stops fighting and starts thinking amidst all the chaos] I'm not odd, I'm eccentric.
[Don is fighting another mutant and he gets near Mike]
Mike: [Complaining] Raph thinks I'm odd.
Don: You're just being sensitive. [Continues fighting]
Mike: Sensitive? If there's one thing I hate, its when people call me sensitive! - Foreshadowing: Quarry only remembers that his real name starts with an S. In the sequel episode "Return to the Underground", it's revealed that his name or rather, her name, is Sydney.
- Had the Silly Thing in Reverse: Happens twice.
- Don tries to put the crystal they have to activate the force field that will drive away the other mutants, but it doesn't work. Mike asks if it is upside down and Don pointedly says it is not upside down, but in a desparate attempt to activate the force field, he turns it upside down and it works.Mike: Don't think I didn't see that Donny, It WAS upside-down.
- In the end of the episode, the Turtles find a huge locked door, with a slot that looks like it could hold one of the mysterious crystals. Don inserts the crystal there and nothing happens, so he tries putting it upside down and it works again.
- Don tries to put the crystal they have to activate the force field that will drive away the other mutants, but it doesn't work. Mike asks if it is upside down and Don pointedly says it is not upside down, but in a desparate attempt to activate the force field, he turns it upside down and it works.
- Mistaken for Flatulence: On their way deeper underground, Mikey takes a whiff and asks who cut the cheese. Donny replies the smell comes from the sulfur pool they need to cross.
- Power Glows: The crystals continue to be prime examples.
- Reluctant Monster: Quarry, Stonebiter and Razorfist only attack the turtles because they believe they are affiliated with The Foot. Once both sides realize they're not enemies they start working together.
- Shout-Out: Mikey saying "Zoinks"
- Skewed Priorities: Mikey stops fighting for his life, because he is offended when Raph calls him "odd". He is even more offended, when Don says he is being sensitive.
- Underground City: What Quarry refers to as 'The Forbidden Zone' and 'The Necropolis'.
