Harley Quinn had just ended her relationship with the Joker. After returning to Gotham City, Harley Quinn met a young girl named Cassandra Kane who had a rare diamond belonging to the crime boss Black Mask.He sent the crazy killer Victor Zsasz to kill Cassandra Kane.. But things soon even out for Harley when she becomes unexpected allies with three deadly women -- Huntress, Black Canary and Renee Montoya.