The pilot episode of DC Universe's animated Harley Quinn series revealed a major change to the classic relationship between Harley Quinn and Poison Ivy. While the episode ended with the two as fast friends, it also revealed that the two villains had a prior relationship that was far more professional.
Harley Quinn and Poison Ivy were first teamed-up in the Batman: The Animated Series episode "Harley and Ivy," where the two villains just happened to run into each other while pulling individual heists at the same museum. The two joined forces to get past the police and a beautiful friendship and classic criminal partnership was born. Since that time, Harley was introduced into the world of the comic books and she and Poison Ivy have gone on to be partners in crime and partners in a romantic sense in multiple media, even tying the knot in the reality of Injustice: Gods Among Us. Despite this, the story of just how Harley and Ivy came to meet is rarely retold.
The first episode of Harley Quinn, "Til Death Do Us Part," changes this considerably, with Harley and Ivy becoming friends and confidants over the course of the episode as Harley makes the decision to change her life and break away from The Joker. The episode reveals that Harley and Ivy had a previous relationship, with Dr. Harleen Quinzel having been Pamela Isley's psychiatrist at Arkham Asylum before she was seduced by Joker and became his sidekick. Another notable change is that Harley is described as having been a good psychiatrist, when many of Harley's origin stories (like the classic Mad Love) suggested she slept her way through medical school or was more concerned with the fame and fortune to be found as a pop psychiatrist than helping her patients recover.
While she is trying to prove to Harley that Joker doesn't love her, Pamela opens up and reminds Harley that she used to be a brilliant psychiatrist who honestly helped her with her abandonment issues and disdain for humanity. "I can be around people now," Pamela says earnestly. "You know, I mean, I hate it. But I can do it without vomiting." She goes on to say that Harleen Quinzel was the only doctor who ever truly helped her, while showing Harley one of her few photos - a framed picture of Pamela and Dr. Quinzel together.
The episode ends happily with the two women now friends and roommates and Poison Ivy firmly supporting Harley's newfound mission to build a new life for herself as an independent villain on her own terms. It remains to be seen if a romance will bloom between Harley Quinn and Poison Ivy like in the comics. For now, however, it is safe to say that Harley and Ivy are back in business.