"Reversal" starts off with Black Siren speaking with a man at the bar. After flirting, the two move into the alley and start kissing. We find out the man is married and when he asks her to tell him a secret, she smiles and it cuts to the intro.
Oliver and Felicity are having dinner at a high-end restaurant. Oliver is approached by a constituent who says she'll be voting against the "nasty" anti-vigilantism referendum. Felicity's phone goes off and she's called to Team Arrow HQ.
There, Dinah has given them a stream where we see a dead body, it's the man Black Siren was with earlier. Dinah says the medical examiner ruled internal
Rene comes in to tell the group that a new poll says 70% of voters in Star City are against the anti-vigilante referendum. When Curtis finds out Felicity was at dinner, he convinces her to go back and says he'll cover her job.
Elsewhere, Black Siren is meeting with a mysterious man. He hands her a folder with an image of her next target.
Felicity's back at her place and she calls Oliver for "dessert." While waiting, she finds Alena hiding in her apartment, she wants Felicity's help with taking down Cayden James — the former Helix operative who has since broken bad.
Alena tells Felicity that she's seen Cayden's plans, and they involved killing 4% of the world's population — nearly 300 million people.
Team Arrow starts hitting the street to find leads on how to track down Black Siren while Felicity and Alena come up with a plan to hack Cayden. On their way leaving the apartment, they run into Oliver.
Felcity tells Oliver what they're up to and Oliver begs her to tell the rest of the team what she's doing. The pair leaves a disappointed Oliver behind.
In a parking garage, a woman is leaving her workout when she drops her keys. She's approached by Black Siren, who picks the keys up for her.
Curtis is working on a piece of technology — he explains to Dinah that it's something that's able to track Black Siren's sonic power. As he's finishing the explanation, the alarm starts going off as she uses the power.
Team Arrow goes to investigate and after finding the woman dead, Dinah is attacked by Black Siren, who blows her away. After getting up, Dinah is unable to find Black Siren and she rendezvous with the rest of Team Arrow.
Felicity and Alena get to the location they were looking for, it's a rave. They approach security guards and tell them they're here to see a hacker named Amnesiac. When they get in the room and start talking to the hacker, a masked man busts in and knocks out everyone in the room beside Felicity and Alena. The guy takes off his mask and it's Oliver — he followed the two ladies.
Felicity and Oliver are arguing about Oliver's actions. Felicity finds the drive she needs and leaves the
Back at Team Arrow HQ, Dinah brings in the autopsy reports of the people Black Siren's killed. They've all had their fingerprints stolen.
Curtis tells Diggle that he might be able to track where she's been, giving them a location of the location she's operating out of.
Diggle calls Felicity and tries to get her back to the team and she tells him about helping out Alena to take down Cayden James.
Curtis calls the team back together and tells them he has a
Felicity and Alena are busy hacking into Cayden's systems at Helix when Felicity sees Black Siren's
Cayden orders his henchmen to shoot Felicity and Alena and as soon as they're about to, Team Arrow shows up to save the day.
Felicity comes out of hiding and notices blood on her hands, Alena's been shot. Felicity gets back to Arrow HQ and tells them that Alena is stabilized.
Curtis and Felicity are working on finding out Black Siren's endgame and Curtis notices that they've been stealing fingerprints in a ploy to take down the internet. If they can get to The Vault and destroy it, it would affect nearly all technology as we know it.
Diggle orders Felicity to go home and relax after the shooting. At her apartment, Felicity is talking to Oliver about how he was able to continue being a vigilante.
Cayden and Black Siren get to The Vault and it's revealed that it was Cayden who helped get Black Siren off of Lian Yu. At The Vault, they use the fingerprints to disable the security system.
Team Arrow gets to The Vault and Felicity starts working on stopping Cayden's virus from spreading across the internet.
A gunfight breaks out between Cayden's henchmen and Team Arrow. Felicity continues trying to stop the virus when Oliver comes over the comm system — he's assumed the role of Overwatch back at HQ and helps guide Felicity to Cayden's computer.
Oliver locks Felicity in the room where Cayden's computer is so she's protected from the henchmen. She's able to override the firewall and stop the virus from spreading, saving The Vault from blowing up.
Felicity is visiting Alena in the hospital. She mentions that Curtis and
At an airport hangar, Cayden gives Black Siren a device that will block Team Arrow from tracking her sonic powers. Cayden then reveals that he wanted Felicity to take down the firewall so as the not leave a trail. Now that The Vault's firewall is done, he's able to take down the internet himself.
Oliver's over at Felicity's and his phone rings. He hangs up and tells Felicity that it was Slade Wilson calling — and asked for Oliver's help.