The Joker has robbed A High Tech Research Lab and planted a Bomb somewhere in Gotham City. The only clue Batman has to the Jokers where'abouts is a video tape left behind, telling Batman to be at the Amusment Park by midnight. Once there, The Joker steals, and hides the Batmobile. Using Onstar, Batman finds the Batmobile, and captures the Joker.
The original document is available at http://ftp.batmanytb.com/tiki-index.php?page=Onstar+Webisode+1