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After splitting with the Joker, Harley Quinn joins superheroines Black Canary, Huntress, and Renee Montoya to save a young girl from an evil crime lord.

A twisted tale told by Harley Quinn herself: When Gotham's most nefariously narcissistic villain Roman Sionis, and his zealous right-hand, Zsasz, put a target on a young girl named Cass, the city is turned upside-down looking for her. Harley, Huntress, Black Canary, and Renee Montoya's paths collide, and the unlikely foursome has no choice but to team up to take down Roman.

After her painful breakup with Joker, shortly after the battle with the Enchantress in Suicide Squad (2016), the already unhinged and extremely volatile Harley Quinn finds herself all alone. But with her mentor out of the picture and a huge target on her back, Harley locks horns with Gotham City's ruthless crime lord Roman Sionis, who's trying to track down a precious diamond and the audacious pickpocket, Cassandra Cain. Now, on the hunt for the teen street hustler and the gemstone, Quinn crosses paths with singer Dinah Lance, Detective Renee Montoya, and a mysterious crossbow-wielding murderess who are all also after Cassandra, as Roman's top assassin, Victor Zsasz, is bent on getting her first. Can Harley take down the brutal super-villain known only as the Black Mask?—Nick Riganas

In Gotham City, the Joker breaks up with Harley Quinn and without his protection, several criminals that have a beef with her hunt her down. When teenager Cassandra Cain pickpockets a diamond that has the account numbers of a fortune from the gangster Victor Zsasz, who is the right-hand of the powerful criminal Roman Sionis, he put a contract on her. Harley Quinn, Police Detective Renee Montoya, Helena "The Huntress" Bertinelli. and Dinah "Black Canary" Lance team up to protect Cassandra.—Claudio Carvalho, Rio de Janeiro, Brazil

Harley Quinn (Margot Robbie) tells her story from the beginning through an animated intro. As a child, her deadbeat alcoholic dad tried to get rid of her multiple times until he finally dumped her at a Catholic girl's school. Harley was always a troublemaker, but she got educated and earned a PhD, becoming a psychologist. That's where she met The Joker and fell in love, leading her to go from Harleen Quinzel to Harley Quinn.

Sometime after the Enchantress' defeat, the Joker breaks up with Harley Quinn, throwing her out on the streets of Gotham City. She is taken in by Doc (Dana Lee), the elderly owner of a Taiwanese restaurant, and recovers from her relationship by staying alone in a crummy apartment above Doc's restaurant, cutting her hair, adopting a spotted hyena (whom she names after Bruce Wayne), and taking up roller derby.

One night, Harley gets drunk at a nightclub owned by gangster Roman Sionis (Ewan McGregor), and cripples Roman's driver after he insults her for dancing on stage. Roman does nothing because Harley hasn't told anybody that she and Joker are broken up, since them being a couple gave her immunity from the wrath of Gotham's other thugs.

Harley is out with other women whom she overhears saying that she's nothing without Joker and that she'll go running back to him soon enough. That night, Harley blows up the Ace Chemicals plant where she pledged herself to the Joker years prior as a way to publicly announce their breakup. When everyone finds out she is not under the Joker's protection anymore, Harley starts getting attacked by every criminal that holds a grudge against her.

Harley then meets burlesque singer Dinah Lance (Jurnee Smollett-Bell), who later rescues an intoxicated Harley from an attempted abduction. Dinah had been singing at Roman's club for years. Spotting the scuffle from his bedroom, Roman is impressed by Dinah's skills and appoints her as his new driver.

Meanwhile, GCPD Detective Renee Montoya (Rosie Perez) investigates a series of mob killings carried out by a crossbow-wielding vigilante. Finding Harley's necklace at the scene of the Ace Chemicals explosion, Montoya notes that Harley is in danger without the Joker's protection as a lot of people in Gotham city want her dead. Montoya approaches Dinah about being an informant on Roman for them, but Dinah rejects the offer, as the GCPD wasn't there to prevent her mother's death. Montoya knew from the previous driver that Roman was expecting a shipment of a special diamond.

Harley is getting her "perfect egg sandwich", but Montoya and a few other angry crooks find and go after her. Montoya tackles Harley and causes her to drop her sandwich into the street, which devastates Harley. When it seems like she's cornered, she throws a bag of garbage at Montoya, which blindsides one crook and causes him to shoot an incoming truck-driving crook, allowing Harley to get away. Unfortunately, Harley runs into a large thug that she and Joker face-tattooed, but before he can get her, he is killed by Helena on her motorcycle. Harley cannot recognize who her savior on the motorcycle was. Harley is then surrounded by Roman's thugs.

Roman always hated Harley. When he finds that she is broken up with the Joker, he asks Victor to round her up for him. Victor is a psychopath who carves a tally on his body for every one of his victims. A week earlier at the club, Roman was telling Mr. Keo (Francois Chau) about the Bertinelli Diamond, which contains codes to their massive fortune (stashed in offshore accounts) and is now going to be in Roman's possession as he knows its location. Roman wanted Keo to partner with him and bring the Golden Lion's family assets to the partnership which would help him control Gotham. The diamond was also the reason Helena's whole family was slaughtered. Keo refused to work with Roman, which is what later leads to the demise of him and his family.

Roman sends Dinah and his sadistic right-hand-man Victor Zsasz (Chris Messina) to retrieve a diamond embedded with the account numbers to the fortune of the Bertinelli crime family, who were massacred years ago. Together, they pick the Diamond from the customs house. Just outside, Young pickpocket Cassandra "Cass" Cain (Ella Jay Basco) steals the diamond from Victor but swallows it after she is arrested for other crimes.

Montoya returns to the police station where she is mocked by other officers. She meets with Captain Patrick Erickson (Steven Williams), who was her former partner that took credit for her hard work and got promoted, and District Attorney Ellen Yee (Ali Wong), who is also Montoya's ex. She is building a case against Roman but doesn't have much support from either of them. Montoya says that Roman is going to be in possession of the Bertinelli Diamond, as she got a tip from his driver.

Montoya is then brought to Cassandra Cain, a young girl brought in for pick pocketing. Montoya then gets a call from Dinah Lance, saying that Cass has a diamond with her. Dinah knows Cass because they live in the same building, and Dinah has a soft spot for Cass since she knows she lives with abusive foster parents.

Harley, fleeing from Montoya and several other people she had wronged in the past, is captured by Roman's men. Victor informs Roman that Cassandra has the diamond. As Roman tortures Harley and prepares to have her killed, she offers to recover the diamond for him. Roman agrees but also places a bounty on Cass to shake up the competition. Breaking into the GCPD with a variety of firework-inspired non-lethal grenade launcher rounds, Harley frees Cassandra and the pair escape to the evidence warehouse. They are ambushed by a number of thugs but manage to kill them all.

After escaping from the evidence warehouse, Harley and Cassandra bond while hiding out at the former's apartment as Harley realizes that she will need to wait for Cassandra to pass the diamond through her intestines before she can go back to Roman. Doc is approached for information by the "crossbow killer", who is revealed to be Helena Bertinelli (Mary Elizabeth Winstead) a vigilante who is the orphaned daughter of gangster Franco Bertinelli.

After her family was murdered by mob boss Stefano Galante (Roberto Catrini), one of Galante's goons saw that she survived but took her away to Sicily to stay with his brother and father. They trained Helena to become the killer that she is now. Having survived her family's massacre and becoming trained as an assassin, Helena has been targeting each of the gangsters responsible for her family's murders, preferring the moniker of "The Huntress". She killed Galante, and the mob guys from earlier were the firing squad that killed her family. Victor is the last name on her list.

Quinn's apartment is later bombed by criminals looking for Cass, and Doc sorrowfully reveals that he betrayed Harley, selling them out for money. Harley calls Roman and offers to turn Cassandra over in exchange for his protection, agreeing to meet at an abandoned amusement park (mostly one of the Joker's former hideouts). Dinah notifies Montoya of the rendezvous, while Victor notices Dinah's treacherous text message in route to the park and informs Roman. Devastated by Dinah's betrayal, Roman dons his ritualistic mask from which he gets his villainous nickname, "B***k Mask".

At the park, Montoya confronts Harley, but Harley knocks her out of a window. Victor arrives and tranquilizes Harley before holding Dinah at gunpoint, but he is killed by Helena, who reveals Victor was the last of her family's killers. Montoya returns and a stand-off ensues, until they realize Roman has arrived with a small army of masked criminals, known collectively as the False Face Society. The ladies then confront each other until they see Roman show up outside with a whole army of thugs. Knowing that he has a bone to pick with all of them, Harley suggests they team up to fight them all. They reluctantly agree and grab a bunch of old weapons (mallet for Harley, bat for Dinah, brass knuckles for Montoya, etc) and head off to fight. Using Quinn's old gear, the makeshift team successfully withstand and repel their attack.

During the battle, Cass is captured by Roman, while Dinah reveals her meta-human ability of supersonic-level screaming (B***k Canary), defeating an additional number of mobsters aligned with Roman. Harley gives chase on roller skates, and with assistance from Helena, the pair pursue Roman. At a nearby pier, the final confrontation occurs. Taking aim at a shadowy figure in the distance she believes to be Roman, Harley wastes her last bullet as Roman appears from behind the figure, which is revealed to be a statue. Roman holds Cassandra hostage and prepares to kill her. However, Cassandra pulls the ring from a grenade she had taken from Harley's weapons chest earlier, slipping the grenade in Roman's jacket. Harley throws Roman from the pier just before the grenade detonates and kills him.

In the aftermath of destroying Roman's criminal empire, Montoya quits the GCPD. With the money within the accounts hidden inside the diamond, she joins Dinah and Helena in establishing a team of vigilantes - the Birds of Prey. Harley and Cassandra escape, selling the diamond itself to a pawnshop and starting their own contract killing business.

Harley is about to reveal a secret about Batman, but the film ends mid-sentence.