Batman and Nightwing discover that Poison Ivy and Floronic Man have teamed up. Much to their reluctance, they decide to look for Harley Quinn (Ivy’s best friend) to find out where the duo are hiding; however, Harley has gone off the grid since getting paroled. Batman leaves finding Harley to Nightwing, while he gets information from A.R.G.U.S.. At A.R.G.U.S. HQ, Batman learns that Ivy and Woodrue stole the information on Swamp Thing’s birth, and kidnapped a scientist who could help them replicate the process and turn everyone into plant people. Nightwing finds Harley waitressing at a restaurant. Tailing her, Nightwing angers her since she wants to live a normal life, but her past as a super criminal keeps her from getting hired as a psychologist. Batman tracks them down, with Harley persuaded in exchange for help getting a good job.
Harley leads them to Slaughter Swamp, in Bludhaven, where Ivy is saddened to see her friend helping Batman. Woodrue fatally wounds the scientist to keep him quiet, as he and Ivy escape; however, the dying man reveals that the duo are heading to Lousiana, where the correct swamp chemicals are. Contacting A.R.G.U.S., Batman, Nightwing and Harley head after them to convince Ivy to stop. Harley resorts to crying to convince Ivy, who is finally swayed by their friendship. Swamp Thing appears, informing Woodrue that he would threaten the Green with his concoction; however, he will not interfere. Batman and Nightwing wonder how to stop Woodrue, until Harley points out he’s a plant and asks if they have a match. Grateful, they both give her a kiss on the cheek and set Woodrue on fire.
Post-credits, Harley has gone back to being Dr. Quinzel. However, she now has a reality game show, where she makes her patients face their fears.