The four Turtles were exposed to the mutagen when after a traffic accident with their young owner as a bystander. The mutagen mutated them into a more human form and more developed intellect. Splinter, a rat once owned by Ninjutsu master Hamato Yoshi, was also exposed. Splinter was fantastically smart and learned Ninjutsu by mimicking his master. Hamato Yoshi moved from Japan to New York in order to escape the bitter love triangle between him and Oroku Nagi, another member of his ninja clan. However, Yoshi was found and killed by Nagi's younger brother Oroku Saki. Saki was the leader of the New York branch of the Foot Clan and had taken the identity The Shredder. Yoshi's death had left Splinter homeless.
