When Batman and Robin get a tip that Commodore Schmidlapp is in danger aboard his yacht, they launch a rescue mission using the Batcopter. As Batman descends on the bat-ladder to land on the yacht, it suddenly vanishes beneath him. He rises out of the sea with a shark attacking his leg. After Batman dislodges it with bat-shark repellent, the shark explodes. Batman and Robin head back to Commissioner Gordon's office, where they deduce that the tip was a set-up by the United Underworld, a gathering of four of the most powerful villains in Gotham City: the Joker, the Penguin, the Riddler, and the Catwoman.