The power is out in Gotham (again, see Forever Evil #1) and Poison Ivy is having a nice stroll through the city, leaving her plants in her wake. She comes across a woman being chased by the father of her daughter, who is hiding in some nearby bushes, and while Ivy deals with the man in her own special way, it triggers some memories of her own childhood.
As with most Gotham villains nowadays, we find out that Ivy didn't have a great childhood. She had a skin condition that caused her to spend a lot of time out of the son, but her mother wanted her to live her life like a normal little girl... Her father on the other hand didn't see it that way, and used his fist as an argument method. But, Ivy's father always bought back her mother's forgiveness with gifts like flowers, which were all planted in a large garden in their yard. But things didn't go all that well, and deteriated to the point where Ivy's father accidentally killed his wife, and burried her body in the garden, only to be caught and arrested.
Fast-forward the origin a bit, and we find Ivy in college, where she experiments with early forms of her pheromone powers, making them pills she'd sell on her campus. She got caught, arrested and was on the verge of expulsion, but she used her one call to contact the dean, and used her womanly charms to get out of trouble, and eventually go on to graduate summa cum laude.
With her life on track, so to speak, Ivy decides it's finally time to confront her father in prison. When he's sat down across from her by the guards, neither of them speak or do anything, until Ivy jumps over the table to give her father a kiss on the cheek. As one would expect, physical contact isn't allowed in visitation, so her father is dragged off, and dies mysteriously from heart failure a day later.
Next on Ivy's career path, she scores an internship at Kane Chemicals, a division of Wayne Enterprises. Eventually she gets a meeting with Bruce Wayne himself, by using her pheromone research to grease the wheels and get her in his office, something usually no first year intern could accomplish. She points out this fact, and pitches Bruce on using her pheromones as ways to influence people to say buy a product, effectively cutting the need for an advertising department. Obviously, Bruce finds some moral issues in Ivy's pitch, which eventually leads to her firing, and finding out that the contract she signed means they own her research. As she's being forcibly escorted off the property, she attempts to gather her notes, and in a struggle, gets doused in all her chemicals.
And that was that, Poison Ivy was born, and now with Gotham in chaos, she uses the opportunity to cover the city in her plants, and proclaim Gotham as her own green kingdom.