Taking place before the events of Batman: Arkham Knight, Harley Quinn breaks into the overgrown Blüdhaven Police Department to recruit Poison Ivy to be part of Scarecrow's plan to destroy the Batman. She searches around the building with the Penguin's help to find out the location of Ivy's cell. After taking out numerous officers, she finds a computer with security camera footage and finds that Ivy is in the basement.

Harley makes her way down to the basement using an elevator, only to be stopped suddenly by the remaining officers. She exits through the top of the elevator and finds that the basement is protected by more officers. By aggravating Ivy's plants to swallow the cops, she is able to secure the basement and continue forward. She finally finds Ivy, but is halted by Nightwing. After dodging his attacks, she defeats him and he is taken away by Ivy's plants. The pair then leave for Gotham City to meet Scarecrow.