Where do Battle Nexus fighters go when they lose?

You can bet it’s not very pleasant, as one of our Ninja Turtles heroes discovers when he’s transported there after a brutal bout. But what he finds is something he never expected: himself. Meanwhile, Donatello is pitted against Metalhead, a robotic Ninja Turtle from another universe! Originally designed by a version of Donnie to help the TMNT become the best fighters they could be, this Metalhead has been reprogrammed to use all of Donnie’s brilliance against him! How can one Ninja Turtle hope to defeat a state-of-the-art machine that knows every fighting style in existence?!