A chemical spills on Bruce Wayne, giving Batman super strength in his right arm.
While chasing some crooks through a construction site, the criminals use a wrecking ball to try and stop Batman and Robin. Surprisingly Batman is able to punch it out of the way, and realizes that he has a super-human punch. Reflecting back to earlier that day, Batman -- in his civilian identity of Bruce Wayne -- was visiting a science lab when he accidentally had an experimental formula spilled on his hands.
Batman then uses his super punch to help round up the crooks that he's chasing before the power wears off. At the end of the whole episode, the scientist that accidentally spilled the chemical on Wayne's hands properly deduces that Bruce Wayne and Batman are one in the same, but decides to do nothing with his knowledge as he wishes to respect Batman's privacy.