The Derby was a high stakes poker game played regularly by high class criminals in Gotham City.
The Derby had been around for decades, even before Bruce Wayne's time. It was well guarded and robbing it was generally regarded as not a good idea. Melanie Walker tried it nonetheless, hoping to rescue her family from what she thought was a Jokerz kidnapping.
Batman would tip off the criminals about the perpetrators. They appear and confront the Royal Flush Gang, with the goal of murdering them for the thefts. After a brief and intense chase, the police arrived, having been informed by Batman, and arrested the Walkers and the criminals had no choice but to let them go; unable to get their revenge nor their money.