After returning to his own body from his temporary stay in Metalhead, a digital imprint of Donatello's mind was left behind. Donatello had found Metalhead and reconstructed it from the damage taken after the siege on Shredder, only to discover another "him" still existing in the robotic body. Michelangelo dubs this second version of his brother "Metal-Don," being the only one besides Donatello to see the digital duplicate as a real person.
Donatello runs a number of tests on his mind's digital duplicate, but Metal-Don becomes increasingly frustrated. Don takes Metal-Don out on the town to clear both of their minds, and are enjoying themselves until they reach the Central Park Zoo, when an encounter with a rhinoceros causes Metal-Don to flash back to the near-fatal beating Donatello suffered at the hands of Rocksteady and Bebop. Metal-Don blasts and kills the rhino, saying that it was better off dead than imprisoned, and expresses a need for his own freedom. He attacks Donatello, demanding his body "back," but the sight of Donatello's tears and pleading once again remind him of Rocksteady and Bebop. Instead, he flees.
Donatello returns home to tell his brothers what happened. They track Metal-Don to one of Harold Lillja's old labs, where they find the empty Metalhead chassis. Donatello looks into the security footage and finds that Metal-Don uploaded himself to a secret project Harold had been working on - a much larger, more powerful Metalhead body.
The Turtles follow Metal-Don to the TCRI building, where he has knocked out all of the guards. Metal-Don hooks himself up to the power core in hopes that the power surge will reboot his CPU; Donatello, however, is worried that not only will the surge destroy Metal-Don, but the entire building as well. Metal-Don and the Turtles scuffle, but the robot initiates his self-destruct sequence. Instead of actually detonating, however, Metal-Don revealed that he purged the "virus" from his system - essentially, the compassion, empathy, and all other humanistic qualities that made Donatello more than a cold, calculating mind.
The robot rockets off into the night, proclaiming that he is no longer Metal-Don.