Let's not kid ourselves. As much as Batman has been used for merchandising, it's always been that the toys were based off the character. With BATMAN UNLIMITED, it's clearly a case of the story being designed to foster a base for the merchandising. It's an advertisement wrapped in entertainment.
The plot: Mr. Freeze (ODED FEHR) is providing asylum to Penguin (DANA SNYDER) at his arctic retreat. But drilling in the region prompts Freeze to respond by siccing a giant monster of his own creation on the crew.
That Freeze can do this gives Penguin an idea, and he encourages Freeze to return to Gotham City and turn it into an icy wasteland for himself, while creating an impossible enemy for Batman. But first they must break out some Arkham inmates (which do not include The Joker, although he gets two cameos voiced by Troy Baker).