Black Siren has returned and now sporting a brunette wig to target a married lawyer and picks him up at a bar. They make out in an alley way and she kills him with her sonic cry.
At a restaurant, Oliver and Felicity are on a date and things are going well for Olicity. A woman at the table near by mentions voting against the anti-vigilante legislation. In fact, it looks like most of Star City is for Team Arrow and their ilk.
Olicity’s dinner is interrupted by Curtis about the murder but since it seems something he can figure out, he sends Felicity back home.
At Felicity’s, Alena from Helix pays her visit asking for help because Cayden James is making something that is very dangerous that could endanger the world’s population. Felicity is working on hacking Cayden’s servers to find out what he is up to. Felicity figures out a way to trick Cayden’s system into think they are them
Oliver drops by after Felicity forgot that she did so and he begs her to tell the team what she is up to.
Later, Black Siren meets with someone who how hands her a file that is a photo of a young woman most likely to kill her using the same MO of the first guy. The woman was a college librarian. Curtis is tasked at finding Black Siren by tracking her sonic cry.
Alena and Felicity go to a rave to speak to some sort of dealer called Amnesiac. The bouncers are scary looking dudes but Amnesiac looks like an account. They ask for the Ghost Drive but the guy thinks they are Narcs. The meeting is interrupted by some guy in a gas mask who takes out the Amnesiac and his goons. It ends up being Oliver being over protective and Felicity is pissed that he messed up everything. The two argue but Felicity tables it so she can help Alena find Cayden James and stop whatever he is planning.
Laurel meets up with her next victim in a parking garage, a young woman named Jenny but she introduces herself as Dinah. Team Arrow figure out where Laurel is and we get another Black Canary versus Black Siren battle but again Black Siren gets away.
Unfortunately, there is no connection between the victims, who Team Arrow note are unremarkable and find out that Laurel is stealing their fingerprints. Diggle figures out they are targets because there is nothing to them and that they are hiding secret identities. Diggle needs felicity to help to find out. He calls Felicity who tells him she needs to help out on a Cayden James matter.
Curits has found Laurel’s HQ to hopefully find out who is their next victim. Unfortunately, Diggle recognizes one of them and sees that Felicity s the next victim at Helix’s former HQ. The team go to her but Laurel gets there first and uses her sonic cry to disrupt their work and we finally meet Cayden James and he is played by Michael Emerson. Alena was the real target and just as his men were going to kill her, Team Arrow arrives and we see a battle ensue between them, the goons and Black Siren. In the aftermath, Alena is badly shot but is stable.
Felicity finds out that the three people killed have access to the International Domain Name Registry and their finger prints are needed to bring down the Internet thus shutting down critical infrastructure. Felicity blames herself and wants to continue working on a solution but Diggle orders her home.
Oliver pays her a visit and tells Felicity is what happened is not her fault. Felicity wonders how Oliver handles everything himself and he said it was her. Oliver tells Felicity that freeing Cayden James at the time was the right decision. Curtis calls to say they located the vault of the Domain Registry and she has to leave.
At the Vault, Black Siren takes out the security guard and we learn that James was the one that rescued her from Lian Yu. Black Siren, James and the man Black Siren met used the finger prints to enter a huge server room.
Team Arrow arrives and Felicity finds out James put a virus to make the whole Internet blow-up. Felicity is working at stopping James while he is working on a way to stop her. James orders Black Siren to take care of Team Arrow. Team Arrow gives her cover so Felicity can do what she needs to do.
Oliver is in the Arrow Cave acting as Overwatch to guide the team to success. Felicity arrives at the main server room and sits at the console desk to do her thing.
Black Siren confronts the new Green Arrow and easily takes down his arrow. Black Siren sees that defeat is nigh and escapes.
Felicity has a moment of doubt but Oliver encourages her and just when things are about to go boom, she manages to stop the virus.
Oliver tells the team they need to figure out why Laurel has aligned herself with Cayden James.
Alena asks Felicity if she can work at her start-up after she finishes finding herself. Alena suggests that Felicity work on mass producing her spinal implant. Felicity then asks Alena if she can use Helix as the name for the start-up. Alena gives her permission and Felicity adds Dynamics to it: Helix Dynamics.
At an airport hanger, James gives Laurel a device so Team Arrow cannot track her sonic cry. Also, James reveals his plan was not to take out the Internet but for the vault’s firewall to be taken down. James wants to have Felicity’s digital finger prints over it rather than his. With the firewall down, James uploaded a code critical to his plan.
Later that night, Felicity and Oliver are having a make-out session that gets interrupted by a phone call by Slade Wilson asking for his help.