A new villain is running around in Gotham, stealing diamonds, using a freeze ray and calling himself Mister Freeze.
Batman entangles him, but Freece proves too powerful.
He tells Batman his origin story. He had been a petty criminal who Batman chased down to a cryogenic lab, where he fell in to one of the sub-zero tanks there, and was electrocuted at the same time. Batman tried to save him, of course.
Fries fell in to a deep sleep.
One day he awoke. He had become a mutant. Able to freece eveything in his path.
He had a professor construct a suit, that keeps his body temperature below zero (Because otherwise he would pass out. And eventually die.) and contains his powers, till he wants to release them.
Freece then goes away, leaving Bats encapsulated in ice. The detectives Yinn and Bennet surprise Batman. He melts the ice with a high tech gizmo, and quickly escapes via Batmobile, shivering violently.
He returns to Batcave and Alfred treats him.
Batman feels guilty, obviously. He thinks about giving up. But soon, thanks to Alfred, he overcomes theses feelings. Alfred has modified the batsuit, with what Bruce can fight back against Freece.
Who continues to terrorize Gotham, and has made an Ice palace. Batman (who has overcome his irrational guilt) fights against him there, and beats him.
An easter egg: In the flashback sequence, before he got superpowers, Fries looks at a photo of a woman who might be Nora Fries. She isn`t seen in the show in any other form.