WARNING: The following contains spoilers for The Suicide Squad, now in theaters and streaming on HBO Max.
Viewers who haven't seen 2016's Suicide Squad can go into James Gunn's The Suicide Squad cold. That being said, it is still in continuity with its predecessor and the DC Extended Universe films that followed. For most of the characters in Task Force X, that doesn't mean much. They're either new to DC's loose cinematic universe or haven't appeared since Suicide Squad. There's one character that's an exception: the first film's breakout star, Harley Quinn (Margot Robbie).
Harley was last seen in 2020's Birds of Prey (and the Fantabulous Emancipation of One Harley Quinn). True to its subtitle, the film was about Harley Quinn striking out on her own after being dumped by the Joker. She teams up with the titular team against their common enemy, mob boss Roman Sionis, better known as Black Mask.
Harley fought alongside Black Canary, the Huntress and Renee Montoya against Sionis. Likely because of her experience with the Suicide Squad, she was a good team player. She was able to work with the Birds in spite of how easily her personality could have clashed with theirs, especially no-nonsense cop Montoya.
The makeshift group was able to defeat Sionis' army of criminals. With an assist from the Huntress, Harley was able to pursue Sionis on roller skates after he kidnapped Cassandra Cain, the young thief Harley had taken under her wing. Harley confronted and ultimately killed Sionis, pushing him off a pier while he was holding a live grenade Cassandra planted on him.
While Harley worked well with the Birds, she wasn't interested in joining the group. As Canary, Huntress and Montoya bonded over tacos, Harley and Cassandra escaped with the diamond that was the film's MacGuffin. To add insult to injury, Harley stole Canary's car to make her getaway. even though she worked well with them, Harley still considers the Birds "dorky little do-gooders." Nevertheless, Harley threw the Birds a bone when she allowed Huntress access to her family fortune via bank codes encrypted on the diamond. Harley pawned the diamond itself and used the money she made go into business with her "apprentice," Cassandra.
It appears that Harley's time as a girlboss was short-lived. In a throwaway line in The Suicide Squad, Rick Flag tells fellow Squad veteran Captain Boomerang that Harley rejoined the team after being caught robbing a bank. Clearly, Harley's start-up didn't work out. She either needed the money or was bored working as a freelance badass. This also calls into question what happened to Cassandra between the end of Birds of Prey and The Suicide Squad. Although none of those answers will be provided in The Suicide Squad, fans will still be happy to see Harley back with the Squad, which requires her winding up back in jail and under Amanda Waller's thumb.
To see how Harley Quinn's story unfolds, watch The Suicide Squad, now in theaters or on HBO Max.