The Turtles must face off against a new enemy, The Rat King, who threatens not only to take control of the city, but Splinter's mind as well.
This episode provides examples of:
- Actor Allusion: The new look of the Rat King (black cape and hat and skull face) makes him look similar to the New Batman Adventures' version of the Scarecrow, whom Jeffrey Combs also voiced.
- Bandaged Face: Rat King. He also seems to be blind, and quite literally sees the world through the eyes of rats.
- The Beastmaster: Rat King
- Body Horror: Rat King is hideously emaciated and corpse-looking, likely having been burned terribly when his laboratory caught fire. His face is almost skull like, with the area around his eyes are blacken.
- Call Back: Dr. Falco returns from "Monkey Brains", and he will never be the same again...
- Close-Call Haircut: Played with. April has exactly two hairs cut off by a mind-controlled Splinter, (thus her model isn't changed) she comments that her bangs were " a little long anyway".
- Continuity Nod: April is shown using a T-Phone like the Turtles in this episode. Her original cell phone (and its first replacement) was destroyed in "Mousers Attack!".
- Dark Is Evil: Rat King's look evokes this to the extreme.
- Evil Counterpart/Not So Different: Rat King tries to paint himself as this to Splinter, saying that they both have lost their humanity, see through the eyes of rats, and eventually lost everything that they stood for.
- From Nobody to Nightmare: Rat King goes from a scientist to a (temporary) empath to a man who controls all the rats in New York City.
- "I Know You're in There Somewhere" Fight: Leo pulls this strategy on a mind-controlled Splinter.
- In Name Only: It doesn't really have anything to do with the original story aside from the introduction of The Rat King, and has more in line with another story featuring him from the 1987 series.
- I, Noun: The title.
- Knight of Cerebus: Rat King attacks the Turtles very personally (trying to murder April and take control of Splinter), and tries to make Splinter give up his humanity via Mind Rape.
- Mind-Control Eyes: Splinter gets these. Also counts as Red Eyes, Take Warning and Glowing Eyes of Doom.
- Mind Manipulation: Rat King has this kind of power over rats.
- Mind Rape: Rat King does this to Splinter while trying to take over his mind.
- Mythology Gag: "I, Monster" is a name frequently used in TMNT media to designate the debut of the Rat King.
- The news broadcast is from Channel Six, the station April worked for in the 1987 series.
- Oh Crap:
: *after brainwashed Splinter beats the turtles
* I just had a horrible realization. He's been going easy on us all these years.
- Running Gag: Mikey is pretty annoyed that the Rat King named himself and spends much of the episode coming up with "better" names for him.
- Swarm of Rats: Rat King wants to take over New York City and has a telepathic connection to rats. You figure it out.
- Trailers Always Spoil: Little was known about the Rat King, and the marketing seemed to make a conscious effort to avoid showing his actor. At least one website, however, managed to get an exclusive clip of the episode, which revealed that he was voiced by Jeffrey Combs, and has a vendetta against the Turtles, pretty much spoiling the identity of the character, Dr. Victor Falco.
- Working Title: "Rat Trap"
- Villain Exit Stage Left: A rather creepy version. Rat King's unconscious body disappears in Swarm of Rats.