The New Adventures of Batman

Birds of a Feather Fool Around Together (1x13)

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Penguin creates the Crime-Slime, which has the capacity to turn anyone evil. He uses it on Bat-Mite, turning him from a harmless imp into a villainous little monster. Whilst trying to return him back to normal, Batman and Robin must also deal with the Penguin and his new creation before any more damage is caused.