The Turtles explore the temple of the lost Muta Kingdom where they free the spirit of light called Crys-Mu, who was trapped inside the Muta Stone. In reward, she gives the Turtles the ability to perform Super Mutation, which turns them into Super Turtles for three minutes. When all four Super Turtles combine into one, they turn into the all-mighty Turtle Saint. At the same time, Shredder and Krang find the Dark Muta Stone, and they try to awaken the demoness Dark Mu contained inside the crystal. (thetvdb.com)