As Leonardo collapses due to the decaying mutagen in his system, Damian returns to the Batcave to find Donatello, Michaelangelo and Casey inside, resulting in a brief fight between them until Batman returns to confirm that the Turtles are his allies. As Damian reports that the League of Assassins are coming to Gotham en masse, Casey reveals that he was provided with equipment that would allow the Turtles to return to their dimension and Batman tells the Turtles to get home before Leonardo's condition gets irreversible. Despite Splinter's warning, Batman and Damian head to Arkham, confident that they can beat the Shredder, only to be confronted by a mass of mutagen-contaminated Arkham inmates (Mr. Freeze as a polar bear, Bane as an elephant, Two-Face as a baboon, etc.), who move in for the attack. During the resulting fight, Batman is trapped by the mutated Mr. Freeze and Poison Ivy. Confronted by al Ghul and the Shredder, he learns that the two archvillains intend to use the mutagen in every major city of the world, throwing human civilization in chaos and thus paving their ascent to total world domination. Watching all of this via security camera in the Batcave, the Turtles decide to stay and risk their lives to save Batman, despite being given the chance to escape back to their own dimension with Casey. The Turtles arrive just as the Joker is about to kill Batman, and while Splinter fights off the remaining inmates, the Dark Knight battles the Shredder inside his latest Intimidator combat armor and the Turtles engage Ra's al Ghul. Although the Shredder severely injures Batman and damages the Intimidator suit, Batman manages to knock his opponent to the ground and Splinter finishes him off. With the Turtles beating him down, and the League of Assassins disarmed and captured by Damian and the G.C.P.D., al Ghul escapes. Just as the Turtles wonder how they will get back home, a dimensional portal opens and disgorges Casey and April O'Neil. The Turtles round up the Foot ninjas and Shredder and send them back to their original dimension. Before they leave, Raphael gives Batman his mask and tells him that he can now count on the Turtles' aid if he ever shows up in their dimension. Batman and Robin return to the Batcave where they learn from Commissioner Gordon that the situation at Arkham is being stabilized until the mutagen has decayed naturally, turning the inmates back to normal within a week.