Joker broke up with Harley Quinn, leaving her feeling somewhat adrift. She had told hardly anyone about what had happened and those few that she did tell didn't really believe her. Stuck in a rut, she decided she needed to something to break out of her slump. She decided to announce the breakup to Gotham City and the world in a highly dramatic fashion.
She traveled to the factory of Ace Chemicals, where she had once fallen into a vat and been changed, just like Joker. There, she spotted a truck and commandeered it, sending it careening into the factory. She sat at a safe distance, watching as the plant caught fire in a burst of colorful fireworks. She left behind a necklace she wore to show her attachment to Joker. The building was leveled.
While investigating another crime, the detective Renee Montoya of the Gotham City Police Department heard the explosions. She found the necklace left in the rubble and began pursuing Harley Quinn. Meanwhile, Quinn soon realized that her statement of emancipation was also unwittingly a declaration of open season for the many who had grievances against her and she found herself being pursued from all directions.