Splinter was formerly a human being, an honorable ninja master named Hamato Yoshi. Yoshi was banished from the Foot Clan in Japan after being deceived by the seditious Oroku Saki, who pinned Hamato Yoshi's dogi to the wall with a knife, preventing him from kneeling before their sensei, which was seen as an insult. When Yoshi removed the knife, the sensei was again insulted, believing Yoshi was drawing the blade in opposition to him. Exiled from the ninja clan, the disgraced Yoshi moved to New York, where he was forced to live in the sewers.
While living in the sewers with the rats as his friends, Yoshi one day found four turtles, recently bought from a pet store by an unnamed boy who accidentally dropped them in the sewer. Yoshi returned one day from his explorations around New York to find the turtles covered with a strange glowing ooze. The substance caused the turtles, most recently exposed to Yoshi, to become humanoid, while Yoshi, most recently exposed to sewer rats, became a humanoid rat, and started going by the pseudonym "Splinter". Yoshi adopts the four turtles as his sons and trains them in the art of ninjitsu. He names them after his favorite Italian renaissance artists: Leonardo da Vinci (Leonardo), Donato di Niccolò di Betto Bardi (Donatello), Raffaello Sanzio (Raphael), and Michelangelo Buonarroti (Michaelangelo). Each Ninja Turtle wears a mask over his eyes with a distinctive color, and is trained in the art of a distinct weapon.
Meanwhile, Oroku Saki has left Japan and tracked Yoshi to New York City, where he intends to destroy him once and for all. He has become associated with Krang, a disembodied alien brain who has been banished from his home, Dimension X, where he was a great warlord. Saki has taken on a new persona, donning a suit covered with razor spikes, complimented by a long cape, and a metal mask over his mouth. He has also taken on the pseudonym "The Shredder".
It becomes clear in the first season that the mutagen that transformed the Turtles and Splinter into their new forms was dumped into the sewer by Shredder in an effort to destroy Yoshi. Shredder thought it was a deadly poison. The Turtles vow to take revenge on the Shredder for dishonoring their master, as well as turning him into a rat. The Turtles want to force him to turn Splinter back into a human again, though this quickly evolves into stopping Shredder's ongoing criminal career with the aid of Channel 6 reporter April O'Neil. The Turtles begin to take on the role of vigilante crime-fighters operating outside of the jurisdiction of law enforcement against any criminals, much like Casey Jones in the third season.
For the first couple of seasons, it seems as if the Turtles are constantly preoccupied with hiding their existence. This seems to be slowly relaxed and, by the last few seasons, most citizens seem to be well aware of them. They also frequently have to deal with citizens misunderstanding them, thanks to the efforts of Burne Thompson, April's employer, and Vernon Fenwick, a Channel 6 cameraman, who distrust the Turtles and frequently blame them for the trouble that the Shredder and Krang cause.
Shredder, Krang, Bebop and Rocksteady (two street thugs morphed into animal form by exposure to the Shredder's mutagen), and a small army of robotic Foot Soldiers try to destroy the Turtles and take over the world. Much of their quest for world domination hinges on bringing the Technodrome (Krang's mobile fortress, and his and The Shredder's base of operation) to the surface as it was either stuck in the earth's core, Dimension X, the arctic, or Arctic Ocean.
In the last three seasons, the show, which had already lasted well past the average lifespan of most Saturday morning cartoon series, went through dramatic changes. The animation became darker and closer to the movies' style, the color of the sky in each episode changed from the traditional blue to a continuous and ominous dark-red sky (which was commonplace with newer action-oriented children's programming at that time), the theme song was changed, the introduction sequence added in clips from the first live-action film, and the show took on a darker, more action-oriented atmosphere.
The Turtles finally banish The Shredder and Krang to Dimension X at the end of the eighth season. They destroy the engines and the "trans-dimensional portal" of the Technodrome preventing them from returning to Earth, though he appears in a three part season ten episode "The Power Of Three". A new villain, Lord Dregg, an evil alien warlord, appeared as their new chief nemesis for the final two seasons. Lord Dregg begins a propaganda campaign against the Turtles, turning the general population against them and in favor of him and his forces. However, Dregg is eventually outed as a villain and the Turtles are finally hailed as heroes within the city. The turtles also suffered from severe mutations that would turn them into big hulks and lose their intelligence temporarily. Also the TMNT acquired a new ally, Carter, a black male with an incurable mutation disease before he left to look for a cure in the future. In the last episode of the series, the Turtles trap Dregg in Dimension X.
