From the introduction:
Welcome to Arkham Asylum! Arkham Asylum is where all the crazies of Gotham get sent to, along with a few of its supervillains. People like the Joker, Mr. Freeze, and Poison Ivy make this place their semi-permanent home. That is, whenever they’re not out making trouble.
Within a few hours, Joker will arrive at the Asylum, escorted by an unknowing Batman, to collect the TITAN formula he forced one Dr. Penelope Young to create. He’ll break most of Arkham’s inmates out of their cells, conscript them into his gang, and begin an riot that’ll leave hundreds or thousands dead in his search for the formula.