The Turtles attempt to use the pieces of the Vortex Crystal from the previous episode to cure themselves of their out-of-control mutations. After undergoing the attempted cure, Michelangelo begins to mutate, larger then before and now able to fire energy beams from his wrists. The other Turtles and Carter attempt to hold him off with no success, as Michelangelo destroys his brothers' weapons and breaks free of attempts to restrain him. His ever-increasing body size causes a cave-in.
It all turns out to be a virtual reality simulation of what would have happened if they were to use the ray on themselves. Splinter advises that stormy seas grow calm when the wind stops blowing, and that they should calm their mind and find the answer. Donatello will run on another test.
Meanwhile on a far-out planet. Batmen are repairing the Dreggnought, while Lord Dregg and HiTech are observing the sensors for the Vortex Crystal. Dregg states the Turtles had discovered that the crystal has the power to stabilize their mutations, but due to low readings, they have only a few fragments. With a tiny piece Dregg can genetically recreate another crystal then complete his Vortex Transporter. His other assistant Mung reports that the Dreggnought is ready for departure. HiTech tells Dregg even they return to Earth it will take months to construct the transporter, and Mung shows him a container filled with over a million computerized Microbots, which can build whatever Dregg desires in only hours. Dregg then offers Mung a position as his second-in-command, which outrages HiTech. Dregg does not care and launches a screaming HiTech off on a probe mission.
Back at the lair, Carter adjusted the ray to match the Turtles genetic codes, which has a simulated success on the Turtles but not Carter.
The Dreggnought arrives near Earth and soon finds that the Vortex Crystal is with the Turtles. Mung sends one Microbot to Earth to get a piece of the crystal, on a tiny stealth rocket. At the lair, Raphael, Donatello, Michelangelo, and Leonardo are wired for the de-mutation ray while Splinter and Carter get the controls ready. The Microbot makes its way to the lair and steals a piece of the crystal while Leonardo was being stabilized. The Turtles normally mutate when they are overly stressed, so they spar to make sure they are cured. In a while, Carter decided to return to college to continue his studies in bio chemistry to find a cure for his mutation. Leonardo shakes hands, but noticed his hand is glowing, and his body mutates even larger then before.
Raphael initially thinks that Leonardo has been messing with the VR simulator, but Donatello reveals that he really did mutate. The mutated Leonardo trashes the lair and escapes, and the other Turtles pursue him while Carter tries to figure out what went wrong.
The micro rocket returns to the Dreggnought with the Microbot and a fragment of the Vortex Crystal. Dregg regrows the crystals and will use it to power his teleporter that send anything to one end of the galaxy to the other, as well to destroy the Turtles.
Back at the city, Leonardo rampages the streets and the Turtles attempt unsuccessfully to calm him. Raphael asks April to keep them off the news, but it turns out that they are being covered live. At the Dreggnaught, Dregg sees that one of the Turtles is on a rampage, and prepares to come back to Earth. Back on Earth, the Microbot container lands near a plant, and after they drives away security, they freely consume all equipment and begin assembling the Vortex Transporter.
When the other Turtles finally find Leonardo sleeping in a sandbox, they are attacked by Dregg and his soldiers, causing Leonardo to rampage once again. He clashes with Carter in his mutant form, and Carter is able to subdue Leonardo in time for the other Turtles to arrive.
April stumbles across the Microbots consuming a hydroelectric generator, and Dregg as well, who decides to make her the first subject for his transporter. Fortunately the Turtles arrive in time to save her, and in the fight Leonardo is able to destroy the robot that Mung created to beat them. However, he is stunned by Dregg, who manages to get a piece of Leonardo's mutated shell. As Dregg and his followers escape, Carter overloads the transporter to keep them away from Earth, which destroys the plant.
At the Dreggnought, Mung apologies for his failure, but Dregg had all planned out. The mutated turtle shell piece he obtained will help him gain unimaginable power. He also kept a fragment of the Vortex Crystal and one Microbot; with the use of the reproducing Microbot, he can build another Transporter and conquer Earth.
At the lair, Leonardo is kept in a wooden cage. Carter decides to remain with the Turtles and find a cure. Raphael agrees because they need a lot of help taming the mutated Leonardo, and he then puts up a sign says "Do not feed the mutant".
- Leonardo (Cam Clarke)
- Donatello (Barry Gordon)
- Raphael (Michael Gough)
- Michelangelo (Townsend Coleman)
- Splinter (Peter Renaday)
- Carter (Bumper Robinson)
- April O'Neil (Renae Jacobs)
- Lord Dregg (Tony Jay)
- Mung (Cam Clarke)
- HiTech (Michael Gough)
- Turtle Van
- Carter's motorcycle
- April's car
Home media releases
- Teenage Mutant Ninja Turtles Volume 1
- Teenage Mutant Ninja Turtles: The Complete Final Season
- Teenage Mutant Ninja Turtles: The Complete Classic Series Collection
- Dregg uses the phrase "2,000 light-years" to mean a space of time, but light-years are used to measure distance, not time.
- When Carter announces that he is stabilizing Leonardo, his mouth doesn't move.
- As the Turtles find mutated Leonardo outside the cinema, the lettering on the front of the Turtle Van is black instead of white.
- When mutated Leonardo attacks the Turtle Van with a street light, the lettering on the front of the Turtle Van is black again.
- Dregg's microbots approach the side of the security guard's desk twice.
- When Donatello tells Michelangelo and Raphael to tie up mutated Leonardo at the diner, Donatello's voice is heard but Raphael's mouth is moving.
- Right after the turtles break through the wall in the Turtle Van to rescue April, lettering on the front of the Turtle Van is black once again.
- When the Turtles flee Dregg's hideout as it explodes, the lettering on the front of the Turtle Van is - you guessed it - black instead of white.
- Michael Gough replaces Rob Paulsen in voicing Raphael and HiTech for the rest of the series, starting with this episode.
- This episode marks the first appearance of Mung and final for HiTech.