When the Ninja Turtles go to Mount Shasta to chase down the evil earth spirit which attacked them at their home, they meet up with a sentinel of the area called Kutuk. Kutuk takes them to a cave to summon Yang, who appears to them first as a giant dragon. The Turtles are surprised to find out he's the very Yang of the Yin-Yang philosophy.
After reverting back to his true form, Yang realizes that the evil spirit they're tracking is Kurko, and sumises that Yin may be in danger. He finds Yin beaten and battered, and has his suspicions confirmed. Yang tells the Turtles that he and Yin are tethered and cannot exist without one another, and that they must beat Kurko. He takes them to the Transmutador Yin-Yang and transforms them into a giant mutant turtle, El Gran Terrapín.
After El Gran Terrapín defeats Kurko, Yin and Yang recover and send the Turtles back home, safely in their beds.