As the four brothers continue their fight against evil, namely Shredder, Shredder's henchmen, and the Kraang, the Turtles will face a new foe that has a larger impact on their lives then they could expect. Slash, a new turtle is in the midst of the Teenage Mutant Ninja Turtles, will force Raphael to deal with a friend that's not too thrilled with how Leonardo, Donatello and Michelangelo treat Raph. While trying to deal with the never ending torrent of evil coming at them in forms of the Rat King, a mutated fungus, and the Kraang, the four brothers also have to deal with their master, Splinter, being mind controlled. With so much fighting and so many enemies, the Turtles might be in a tight spot but when they get help from the vigilante Casey Jones. This new crime fighter likes to wear a hockey mask and talks more trash then a landfill holds, but the Turtles find that they might be able to trust Casey Jones to fight with them.