Warning: The following contains SPOILERS for Harley Quinn, episode 5, "Being Harley Quinn."
The latest episode of DC Universe's Harley Quinn reveals the true story of how Dr. Harleen Quinzel became the Clown Queen of Crime. This new origin is born, quite fittingly given Harley's training as a psychiatrist, as a result in a change of view rather than a major retcon from one of her earlier origin stories.
When Harley Quinn was first created for Batman: The Animated Series, she was intended to be a one-off character in the same vein as the female companions employed by the villains in the 1966 Batman show. When Harley proved popular and returned in later episodes, her background was expanded, revealing that she had once been The Joker's psychiatrist before being charmed into becoming his henchwoman. This basic idea was retained when Harley was introduced into the comics in 1999, but her origins here also gave her enhanced strength and agility thanks to a special formula created by Poison Ivy. The most recent comics and the Suicide Squad movie changed Harley's background again, with Dr. Quinzel now being bathed in the same chemicals that created The Joker.
This seems to be Harley's history when Harley Quinn, episode 5, "Being Harley Quinn" opens. The action of the episode saw Doctor Psycho taking Poison Ivy, King Shark and Clayface with him into Harley's head after she became physically and mentally paralyzed by indecision. This led them into a psychic landscape resembling a museum, which was made up of Harley's memories. It was here that they discovered that the screen depicting Harley's origin story contained a glitch, skipping over the moment when The Joker pushed Harleen into a giant chemical vat.
Later, while trying to escape from Harley's head, the gang found themselves passing by the part of Harley's mind devoted to repressed memories. Desperate for answers as to why her mind was skipping over such a pivotal moment of her life, Harley went off in search of the lost memory. This led to a shocking revelation: Harleen had jumped into the chemical vat, rather than being pushed. Harley then realized she wasn't letting herself remember the truth because it was far easier for her to just blame The Joker for all her problems than to accept that she had made the decision to become a villain herself.
Thankfully, Harley Quinn had become much more assertive over the past few months, so she stepped into the memory, replacing her past self and read The Joker the riot act before beating him senseless with her baseball bat. Her confidence restored, Harley declared that she didn't become Harley Quinn the night she jumped into the chemical vat; she became Harley Quinn when she first stood up to The Joker and blew up his hideout. The Joker weakly protested that Harley couldn't just decide to change her past but Harley only replied, "My mind. My rules," before turning and leaving the memory without a second thought.