It's a dark and stormy night, and the Turtles are returning home after a disastrous mission, in which they tried to find missing explosives but only found cheese. After hearing a news report about rats, the Turtles' conversation switches to the topic of the Rat King.
In his own domain, the Rat King is raging because there is no food for himself or his rats. He stumbles across a canister of "green junk" (mutagen), but dismisses it as inedible. One of his rats advises him that food could be found for them by "a bigger rat, a stronger rat" like Splinter. He sets out to hypnotize the mutant rat, but has difficulty locating the lair.
However, he does stumble across Bebop and Rocksteady emerging from a portal. They were supposed to meet someone who is selling them explosives, but since they're already down there, Shredder orders them to locate a canister of mutagen that was lost in the sewers. The Rat King overhears that Splinter and the Turtles were made the way they are through mutagen, and decides to mutate some humans.
The Rat King sends an army of rats after Irma and Vernon, and immediately apprehends them. But as he prepares to drench them in mutagen, he notices that it's changed from green to brown, and then to rainbow hues. He proceeds anyway, and both of them turn into giant rats. Irma refuses to take orders from the Rat King, so he hypnotizes them with his flute and commands them to go get food.
While the Turtles are tracking Irma and Vernon, a wall is chewed down by a vast army of rats. The rats then chew the ground out from under them, and it's only by Michelangelo's grappling hook that they avoid being chewed as well. Unfortunately, the rats start chewing the rope as well, forcing the Turtles to propel themselves into a metal hatch.
Vernon and Irma burst out into the streets, where passers-by believe that they are were-rats because it's a full moon. They raid a cheese store and claw their way into the next store, only to discover it's a pet store with several cats. Panicking, they turn back to normal, with no memory of what the Rat King did to them.
The Turtles make it to the Rat King's lair, where he immediately attacks them. They overhear him mention what he did to Vernon and Irma, and decide to go ask Splinter for help.
April returns to Channel 6 and discovers Vernon and Irma have returned. Donatello tells her that they are the giant rats - which is backed up when Vernon transforms on the air. Their mutations only take place when the Rat King plays his flute. Irma also transforms and drags April to the Rat King, who declares that April will be his next super-rat.
Meanwhile, Bebop and Rocksteady head up to a restaurant called the Flaming Turkey, to meet up with the explosives smugglers. The smugglers are planning to switch the explosives into cheese crates and cheat Shredder; Bebop and Rocksteady overhear this and decide to follow the smugglers and get the explosives for real.
The Rat King prepares to make April a rat mutant, when Vernon (who also overheard the smugglers) runs in with news of enormous amounts of cheese at the airport. The Rat King leaves with his new servants. Fortunately the Turtles rush in to save April from being mutated, and Donatello states that the rainbow-colored mutagen has become unstable, which is why the mutations have been so erratic.
At the airport, the smugglers are chased off their plane by Irma and Vernon, who have no idea that the cheese crates are actually filled with explosives. The two were-rats begin fighting the Turtles, and Michelangelo accidentally activates the plane and starts taxiing it down the runway. Bebop and Rocksteady leap on board as well, finding the Turtles being throttled by a pair of mutant rats. Irma and Vernon refuse to let them take the cheese crates, and a three-way fight breaks out among the mutants, even as the plane takes off.
Meanwhile, Splinter attacks the Rat King, intending to take his flute, but ends up mesmerized by it instead. Fortunately, April is nearby, and she splashes Splinter with a bucket of water to snap him back to normal. He knocks the flute out of the Rat King's hand and takes it himself.
Leonardo discovers the missing explosives just before Rocksteady blows out the wall, causing the Turtles and were-rats to plummet out in mid-air. Fortunately, Donatello arrives with the Turtle blimp, saving them. Rocksteady and Bebop fly the plane into the Technodrome, but the plane's controls aren't responding. It crashes, causing a great deal of damage.
Back in the lair, Splinter destroys the flute. On the news, Vernon begins eating cheese during a news broadcast, hinting that his transition back to a normal human isn't quite complete.
- Leonardo (Cam Clarke)
- Donatello (Barry Gordon)
- Michelangelo (Townsend Coleman)
- Raphael (Rob Paulsen)
- Splinter (Peter Renaday)
- April O'Neil (Renae Jacobs)
- Vernon Fenwick (Peter Renaday)
- Irma Langinstein (Jennifer Darling)
- Shredder (James Avery)
- Krang (Pat Fraley)
- Bebop (Barry Gordon)
- Rocksteady (Cam Clarke)
- Rat King (Townsend Coleman)
- First Explosive Thief (Rob Paulsen)
- Second Explosive Thief (Townsend Coleman)
- Gypsy Woman (Jennifer Darling)
- Cheese Shop Owner (Pat Fraley)
- Man in Street (James Avery)
- Turtles' Lair
- Rat King's Lair
- City Sewer System
- City Airport
- Cheese shop
- Pet shop
- Flaming Turkey restaurant
- Channel 6
- Turtle Blimp
Home media releases
- Teenage Mutant Ninja Turtles Season 4
- Teenage Mutant Ninja Turtles: The Complete Classic Series Collection
- Up until this point in the cartoon series, mutagen has always been depicted as a glowing pink in color. Starting with this episode, it is changed to green, as it is typically colored in the vast majority of Turtles media.
- First time Irma and Vernon are turned into mutants, the second being into insects in "Revenge of the Fly".
- Shredder spoke to Rocksteady and Bebop with his Com-Link instead of watching them on the portal's view screen.
- Final appearance of the Rat King's flute until he acquires another one in "Night of the Rogues".
- Donatello is depicted as being in two places at once during the final confrontation: he is depicted as accompanying the other Turtles to fight Rocksteady and Bebop, but he also flies the Turtle Blimp in to the rescue after the climax after declaring that he would meet them at the airport. This may be due to the animators not realizing where he was supposed to be, since he does not speak a word during the fight.
- When Rocksteady spoke to Shredder on his comlink, he wasn't holding the comlink.
- Donatello's belt was colored purple after April asked the Turtles if they'd seen Vernon and Irma.
- When the Turtles leave the Rat King's lair, Leonardo has Raphael's colors.
- The mail box outside Channel 6 that it seen when Rat King goes topside, is misspelled as "US Male".
- If the Rat King really wanted to turn April into a rat, why didn't he just splash the mutagen on her instead of tying her up next to a device that would drip the mutagen onto her?