From the Game's Introduction:
The (almost) all Batman™ game based on an idea that may have originated with Don Ross is a Live Action Role Playing (LARP) game. Each player in the game is given a character to portray. Each character has her/his own goals and personality. In this game there are no winners or losers. Though some characters may have 'goals', actual success at attaining the goals is no reflection on the player's ability. (For example, an actor is playing Shakespeare's Hamlet rather poorly if by the end of his performance the tragic prince is still alive....)
The (almost) all Batman™ game based on an idea that may have originated with Don Ross is based on a simple premise. It is an evening in Gotham, and there are many many characters all in the same place who happen to be dressed up as Batman™.
The villains in Gotham City have promised that tonight they will kill Batman™ at the Penguin's "Iceberg Lounge" Casino. Expecting that their favorite hero would willingly walk into such a trap, many citizens of Gotham decided to protect the dark knight by coming to the Iceberg Lounge dressed at Batman™ themselves, so the villains don't know who the real Batman™ is. Well, in order that they not stick out, the villains are dressing up as Batman™ too.