after moving to new york city, yoshi has just purchased four pet turtles when he stumbles upon an exchange in an alley between two kraang droids. seeking to silence yoshi before he can share what he has seen, a fight ensues and the canister the droids have been carrying smashes open, splashing both yoshi and his turtles with a mutagen. as the result of stepping on the tail of a black rat upon entering an alley, yoshi mutates into a humanoid mutant black rat, while the turtles take on human characteristics. realizing he can no longer live a normal life, he retreats to the new york sewers where he raises his pets as sons and teaches them ninjutsu.