The Joker has set himself free from the Mental Hospital for the Criminally Insane Batman had him incarcerated in, and he is out for blood. Before he was sent away, knowing that one member of his previous gang had ratted him out to the police, he had vowed to murder all five of them; And only the Batman is capable of stopping him. Despite Batman's best efforts, the Joker is able to finish off nearly all of his hit list. Batman is only alerted after the first murder when a grinning corpse is found in the mud just outside of Gotham City, and he goes to each of the former members of the Joker's gang individually to try to protect them.
The second member agrees to go into protective custody with Batman, but drinks from his water bucket right afterward and is killed by Joker Venom it had been laced with. The third member is treated favorably by the Joker and even believes the two of them to be on good terms when the Joker gives him a cigar before leaving his apartment. As the Joker walks out of the building, the cigar blows up the man's entire household.

Joker defeats Batman

Most interestingly, the fourth member of the gang believes Batman is only trying to bring him in for a mugging he recently committed, even after Batman tells him the story. He asks if Batman will lead the way because he's frightened, and then the second his back is turned bashes Batman over the head with a sap and runs back to his hideout. When Batman regains his composure enough to pursue, he finds the man already strung up by the neck, hanged from his rafters, and the Joker lying in ambush. Although the Joker is able to subdue the already disoriented Batman, he declines to kill him, because it was "mere chance that caused his attack on Batman to succeed," and he had "always envisioned his winning as a result of cunning at the end of a bitter struggle."

When Batman regains consciousness, he is able to deduce the location of the Joker's hideout by the strange mixture of sand and oil on his face where the Joker had kicked him. The Joker is hiding out in the aquarium right next to the beach, where there was very recently an oil spill. Batman tracks the Joker down, hoping to save the life of the last man, now a senior citizen confined to a wheelchair. Naturally, knowing Batman would find him, the Joker has the man poised over a giant tank of water with a shark inside and agrees that he will let him go only if Batman takes his place. Batman is forced to agree, and after he is handcuffed, the Joker pushes both of them in. Thinking fast, Batman is actually able to subdue the shark using only his handcuffs and break the glass of the tank by bashing it with the wheelchair, saving the man's life and escaping in barely enough time to pursue the Joker. Batman runs out into the early dawn, and chases the Joker to his getaway car, but worries that in his current condition, after the lump on his head, the lack of air, and the night of running around Gotham; he won't be able to catch up to him in time. But the Joker slips on the oil slime of the beach right before reaching his escape, and Batman is able to catch up with him and beat him silly.

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