As Neron’s evil spreads throughout the world, Arkham Asylum’s owner, Jeremiah Arkham, wonders whether something is affecting his home for the criminally insane: a mass poisoner, Kryppen, is viewed on security tapes talking with his shadow.
In the mess hall, as a thunder storm rages outside the asylum, tensions rise and a fight breaks out, giving one inmate, Bradbury, the opportunity to smash through the windows. He quickly scales the wall and takes hold of the lightning rod on the roof just as it is struck by lightning. A moment later and the entire asylum is plunged into darkness as the power is shorted out.
Trying to restore order in the dark, Jeremiah Arkham is attacked by Kryppen who claims to have made a deal with the devil. In seconds, a full scale riot breaks out but to Kryppen’s dismay, he is caught with the others and taken to his cell. Surprising his guards, he manages to escape and makes his way to the pharmacy.
Despite the power outage, a makeshift dinner of cocoa and cookies is prepared and delivered to every inmate with an orderly delivering Arkham’s personally. After tasting the cocoa, the orderly reveals himself to be Kryppen who has poisoned not only the doctor’s drink, but everyone else’s as well.
With the whole asylum incapacitated, Kryppen puts out a call for Batman who arrives to find everyone dying. The poisoner tells him that, as per his deal with the devil, if Batman kills just one man, whether prisoner or guard, Kryppen will administer the antidote to everyone else as Batman forfeits his soul. Should he refuse to do so, everyone dies.
Surprising Kryppen, Batman refuses and forces him to taste his own poison, figuring that the loss of his soul will be as nothing to what the devil will do to Kryppen’s. Out manoeuvred, Kryppen takes Batman to the antidote which he then spreads around the others. Kryppen, meanwhile, is left terrified of being discovered by the devil as Batman takes to the streets again.