- "Can I help you?"
"Why, yes, yes, you can. I'm here to report a terrible crime."
"And what terrible crime is that?"
- ―A Gotham City Police Department receptionist and Harley Quinn[src]
As part of his play to become the most powerful crime lord in Gotham City, Roman Simonis, also known as the Black Mask, moved to secure the Bertinelli Diamond for funding for his organization. Through his underworld contacts he was able to secure the diamond and send his lieutenant Victor Zsasz to secure it. After he did, it was snatched by Cassandra Cain, who was unaware of its true value as the key to unlocking the fortune of the Bertinelli crime family. She was taken away to be imprisoned in one of the cells at the Gotham City Police Department and swallowed the diamond on the way there.
Meanwhile, Harley Quinn was captured by the Black Mask, having declared her emancipation from the Joker and thus no longer protected by him. Black Mask intended to kill her cruelly, but she overheard him talking about the diamond and offered her services as a finder of lost things. He agreed to release her, but also put out a general reward of $500,000 for anyone who could get him back his diamond.
Harley Quinn began her assault in a stealth manner. She simply walked into the police department disguised as a high-class lady and told the receptionist that she was there to report a terrible crime. When he disinterestedly asked what it was, she replied "This one," and blasted in the face using a paintball gun. She then rampaged through the department, using her gun to create powerful blasts and her martial arts skills to take down others who opposed her. She finally reached the cells and managed to release Cain, but in the process also freed a number of other criminals.
The result was chaos, a free-for-all now featuring her, the dangerous criminals she had released, and the Gotham City Hunters who had now infiltrated the jail, intent on collecting the reward put out by the Black Mask. Quinn had to contend with all of these, while also keeping track of Cain, who was determined to make a break for it, but didn't have the skills on her own to face the other folks who were after her and who were much more dangerous than Quinn. Mustering all of her powers and resources, Quinn battled through the lot, securing Cain and making a break.
Quinn managed to get away, taking Cain to a store to raid for supplies such as laxatives to try to get the diamond out of her. Black Mask was angered that none of the folks he had sent other than Quinn were able to secure one little girl. He was even more ticked when Quinn, awakened to the dangers arrayed against her, called him to renegotiate the terms of their deal, demanding protection from him.