Baxter worked as the evil ninja master's lackey until a horrible incident mutated him into a fly.
Stockman started out an honest life trying to sell his Mousers to the Ajax pest control agency. They did not like his suggestion, seeing that it would work a little too well to rid the city of rats, and thus, put the pest control agency out of business.
So Stockman was thrown into the street. Shredder had watched this via his cameras. He would later find Stockman and offer him the job of creating a Master Control System for an army of Mousers. Shredder, not being able to wait, used a replication device at the Technodrome to assemble twelve Mousers. He then programmed the Mousers to find and destroy Splinter, the mutant rat who was the Ninja Turtles' master. The Mousers were destroyed by the TMNT, and they found Stockman's name on the devices. The Turtles and Splinter found Baxter with April O'Neil's help. While Stockman didn't know Shredder was using the Mousers to kill the Turtles and Splinter, Shredder still sent the Foot Soldiers to eliminate him. The Turtles defeated the Foot and tied Stockman to a lamppost to interrogate him, and he told them his part of the story. He would be arrested and Donatello would use Stockman's van and the equipment he found in his apartment to create the Turtle Van and the Turtle Blimp.
Stockman was later committed to an insane asylum, due to his tales of a giant talking rat and talking Ninja Turtles and has since gone criminally insane. Shredder returned to bust him out and use him to help in his attempts to defeat the Turtles. Baxter, now a lot more evil-minded and insane than he had been before his incarceration, became his sidekick and lackey, helping him to get the "Three Fragments of the Eye of Sarnath", an alien artifact that would grant the owner virtually limitless power. Stockman eventually became fed up with Shredder's abuse and tried to use the Eye of Sarnath for himself once Shredder acquired the three segments of the powerful artifact. The Eye could create things out of thin air, or transmute objects, but it had no effect on gold. Shredder would take advantage of this to get the Eye back, even though he'd still end up getting defeated by the Turtles when Donatello's Sarnathometer destroyed the Eye via a massive explosion. Baxter would crawl back to Shredder's side, but the damage would be done as Shredder began conspiring to rid himself of the turncoat scientist. The final full episode with Baxter as a human would be "The Case of the Killer Pizzas" where he helps Shredder unleash killer monsters from Dimension X to attack the city.
Baxter's big change occurred in the second season episode, "Enter The Fly", in which, after a failed attempt to generate a force field between the towers of the World Trade Center, Shredder decided he required brawn to replace Baxter's brains. Instructing Krang to send his mutant flunkies Bebop and Rocksteady through a dimensional portal to Earth, Shredder was warned that the dimensional interface was unstable, and required that someone be sent back through the portal to maintain the balance. Baxter was elected, and much to the scientist's horror, was hurled through the portal to Dimension X. Krang had no use for Baxter's scientific talents, and simply chose to kill him, tossing him in a disintegrator unit. However, in a reference to the 1986 movie The Fly, a common housefly that had been on Baxter's clothes when he was tossed through the portal was also trapped in the chamber with him, and their molecules wound up being intermingled, cross-mutating Baxter into a giant, humanoid fly-creature. Immediately determined to get revenge for his mutation, Baxter fled Dimension X and attacked both the Turtles and Shredder, but his addled, not-entirely-sane mind was open to suggestion, and Shredder managed to talk him down by convincing him that the Turtles were responsible for his condition (Ironically, the Turtles never recognized Baxter in his new state). Baxter then aided Shredder in a plot to trap the Turtles a micro-second forward in time, forever out of phase with the rest of reality. During the battle that ensued, however, Baxter wound up accidentally flying between the two pylons of the device, and disappeared in a flash of energy.
Baxter returned in the third season episode "Return of the Fly", beginning a series of episode titles that referenced and homages those of typical horror B movies. Having been shunted out of phase with our dimension, the invisible, intangible Baxter could still observe the world around him, and spent months searching the sewers as a wraith, eventually locating the Turtles' lair and forming a plan to get revenge on both them and Shredder at the same time. Realigning his molecules with the rest of reality by allowing himself to be struck by a lightning bolt, Baxter kidnapped April O'Neil in order to lure the Turtles into his trap, but once again, Shredder was able to smooth-talk the addle-brained fly into teaming up with him again. Unfortunately for Baxter, an accident with a freeze ray foiled his scheme and entombed in a block of ice. Sent tumbling into the Technodrome, Baxter burst free from his icy prison just as Shredder and Krang were mocking his ineptitude, and flew out of the base in a rage.
Later in the third season, Baxter reappeared in "Bye Bye Fly", eking out a miserable living in the catacombs beneath the city, feeding off trash. As the episode began, he stumbled across a group of archaeologists, who had unearthed what appeared to be an ancient temple beneath the city. Enraged that his catacombs had been invaded, Baxter chased the archaeologists out and investigated the temple, which, in actuality, turned out to be an inter-dimensional space ship that had crashed on Earth centuries ago. The ship's sentient computer was happy to have some company at last and befriended Baxter, providing him with a trans-mutation gun that allowed him to have his revenge on Shredder by transforming him into an ordinary housefly. Baxter also used the gun on Michaelangelo, turning the Ninja Turtle into a pet gerbil, but Mikey's brothers managed to save the day and return their compadre to his regular reptilian state. The Turtles escaped the ship just before it took off for Dimension X, taking with them a key component of the ship's warp drive, without which the ship disassembled in mid-flight, stranding Baxter in an interdimensional limbo, with a giant alien spider bearing down on him.
Baxter had managed to escape the spider by the time he appeared the next year, in season four's "Son of Return of the Fly II". Although he still had the ship's computer for company, the exile evidently didn't do Baxter's mind any good, as he steadily began to act more and more like a fly, being fascinated by bright lights, craving sugar, being unable to focus on complicated tasks, and not even being able to remember exactly who it was he wanted revenge on, or why. With the computer's help and guidance, he was able to return to Earth through a small rift in the fabric of space-time, and captured the Turtles in order to lure Shredder into combat. Unable to bear the thought of anyone other than him destroying the Turtles, Shredder answered Baxter's challenge, but in the battle that ensued, the computer was destroyed, leaving only a circuit board that housed its intelligence behind. Shredder fled back to Dimension X, but when Baxter attempted to pursue him, Krang shut the portal down before Baxter emerged, again stranding the fly in inter-dimensional space.
Baxter's next appearance, in season five's "Landlord of the Flies", was a distinctly anomalous one. Without the computer at his side or any explanation for how he escaped his inter-dimensional banishment, Baxter was already back on Earth when the episode began, with a new ability: his continually devolving fly-like mentality had given him the ability to communicate with other flies, and he assembled the insects into an army in a plot to get revenge on Shredder. By this time, however, the Technodrome was stranded in the polar wastes of the arctic, and the frigid air proved distinctly inhospitable to Baxter and his fly family, stopping his scheme before it even got started. Krang was intrigued by his new ability, however, and struck a deal with Baxter, promising to transform him back into a human again if he helped them. Baxter proceeded to terrorize the city with his family army, but was one again shot off into another dimension at the episode's conclusion by Donatello's portable portal generator.
In his final appearance, in season seven's "Revenge of the Fly", it was as if "Landlord of the Flies" had never happened: Baxter was in limbo, with the computer's remains, and with Krang credited for trapping him there. It was, ironically, Krang who was responsible for releasing Baxter, when he attempted to open a portal between Earth and Dimension X, and the malfuncioning device wound up opening a gateway to Baxter's prison. Quickly escaping, Baxter plugged his computer friend into the Technodrome's databanks, giving him complete control over the fortress, allowing him to lock up Shredder and his cohorts. Seeking revenge against the entire world, Baxter stole a sample of Krang's mutagen, and combined it with the genetic material of various insects stolen from a research lab, using the resultant concoction to transform humans into mutant insects, including the crew at Channel 6. The computer reminded him that he had forgotten about the Turtles, and he used his insect minions to capture them, but when questioned about what, exactly, they had ever actually done to him, Baxter admitted he couldn't even remember. Eventually, the Turtles were able to overpower Baxter, and forced him to lead them back to the Technodrome so that they could steal Shredder's retro-mutagen ray and return the mutated humans to normal. In the chaos that ensued, the computer was destroyed, and once the retro-mutagen ray had been found, Baxter snatched it up and flew off through a dimensional portal, intending to use it to return himself to normal. While the Turtles gave chase, Shredder attempted to close the portal of the group before they could re-emerge, but the Turtles beat the clock, reclaiming the ray and making it back through in time, Baxter, however, was not so lucky, and was once again lost in limbo, never to appear again.
Besides the Mousers, Stockman was also the inventor of the Knucklehead, which Shredder utilized after he got rid of Stockman.
Baxter Stockman also had a red-haired twin brother named Barney who was also a mad scientist and with the same voice. He threw fits whenever the Turtles confused him with Baxter, hated his brother and the fact that he turned into a fly. Barney Stockman appeared in the episode "Raphael Knocks 'Em Dead".
- In the IDW Comics Stockman's fly form of the series is taken up briefly again in the form of the Flyborgs.
- Stockman was originally meant to become an ally of the Turtles at the end of the first season. However that idea was scrapped by Eastman and Laird.
- It is worth noting that Baxter being white was a change from the comics, where he was African-American.
- A Thing About Rats
- Return of the Shredder
- The Incredible Shrinking Turtles
- It Came from Beneath the Sewers
- The Mean Machines
- Curse of the Evil Eye
- The Case of the Killer Pizzas
- Enter The Fly
- Return of the Fly
- Bye, Bye, Fly
- Son of Return of the Fly II
- Landlord of the Flies
- Revenge of the Fly