The former cop who was convinced that he was Batman (see Batman 402) escaped Arkham after killing two guards who he thought was Joker and Two-Face. He happened upon the Batcave, stole an outfit, and stole the car. The real Batman realized what was going on and tracked the impostor down. After a short fight, the true Batman won.
The continuation of a writeup by Max A. Collins titled "The Dick Tracy/Batman Connection Part Two: The Galleries of Grotesque".