Origin (The Next Chapter)

Oroku Saki and Hamato Yoshi had met as orphaned street kids and started the Foot Clan.

Years later, Oroku Saki and Hamato Yoshi were members of the Foot Clan living in Tōkyō. The adopted brothers were rivals, but they never fought more over anything than over the love of Tang Shen. However, she chose Yoshi, and the two married. They had a child named Hamato Miwa together. A few months after Miwa's birth, Saki came into their home wielding tekkō-Kagi after learning of his true identity (he later gained the title the Shredder) and started a fight with Yoshi. A fire was started, in which Shen perished by no one's intention and Saki became bald and disfigured. Saki stole Miwa in the confusion, renamed her Karai, and raised her as his adopted daughter as he began rebuilding the Foot Clan. Yoshi was excommunicated by the Foot and fled to New York, where he lived homeless in the sewers.

Back in Japan, Saki turned the Foot into an army of crime, using their ninja skills for personal gain.

Later, a five-year-old April O'Neil dropped a fishbowl of turtle hatchlings down a New York storm drain. Yoshi found them and raised them as pets.

Agents from the Techno-Cosmic Research Institute are on a mission to kill Baxter Stockman and steal something that he created: mutagen. The leader of the TCRI Agents, Spider Bytez, a muscular and tough man, tells the agents that the mutagen is capable of merging an animal with a human. Spider Bytez tells the agents to break in, kill Stockman, and steal a vial of a green substance. As the agents broke in, they found Baxter Stockman growing mutagen. Before the agents could shoot their laser weapons at them, Baxter made a deal with them. He would give them the mutagen, and Baxter would be home free. Spider Bytes tells the Agents to hold their fire, he has a "better idea". Spider Bytez says to "take him to Shredder." One of the Agents uses his stun weapon to knock Baxter out. The agents pick Baxter's unconscious body up. Baxter's hand still gripped a vial of mutagen, but he drops the vial. The agents don't notice. The vial rolls over into a storm drain. No one, not even Dr. Stockman himself, could have predicted the chain of reactions that held on that day.

That day, Yoshi discovered the turtles crawling in a broken canister of glowing, green ooze and proceeded to clean them. Yoshi later realized that the ooze was a powerful mutagen, capable of mutating any lifeform by combining its genetic code with the DNA of whatever animal it last touched. As the turtles had last touched Yoshi, they became humanoid. Yoshi, however, had most recently been in contact with his pet rat, Splinter, and thus became a half-man, half-rat mutant.

As ever, Yoshi named his sons after Renaissance artists and taught them ninjutsu, although it was for protection rather than vengeance. The Ninja Turtles began to call him "Splinter" in reference to the pet rat Yoshi merged with, a name that he willingly adopted.