Sensing Master Splinter's despair at his new life as a rat, the Turtles agree to use the small amount of mutagen that they have left to concoct a formula that will revert Splinter back into Yoshi. The experiment is a success and Yoshi decides to take a stroll, as a human, through the city. Meanwhile, April is reporting Public Service Announcements from the local library when spies Rocksteady and Bebop. She immediately contacts the Turtles who head straight for the library where they encounter Shredder. It seems that Shredder has stumbled onto a spell that will enable him to open a portal to any dimension he chooses.The Turtles find a reference to a temple in an old subway terminal somewhere under the city.