4. Reversal

Complete story — includes the ending

Black Siren begins killing seemingly unimportant individuals while Felicity is approached by Alena, who tells her that Cayden James is planning to cause hundreds of millions of casualties worldwide. They meet black market dealer "Amnesiac" to buy a "ghost drive". Feeling concerned, Oliver intervenes and attacks the thugs, angering Felicity, who steals the drive. Team Arrow tracks down Black Siren to the Helix facility and discovers she is working for James, who tasks his men to kill Felicity and Alena, who is critically shot during the rescue by Team Arrow. Felicity learns that James has stolen the victims' fingerprints, necessary to enter the International Domain Name Directory (IDND), a global internet infrastructure hub. Believing that James intends to destroy the internet, Team Arrow attacks his party at the IDND, where Felicity manages to breach through the firewall to stop James' apparent attack. Felicity adds Alena to her startup, calling it Helix and intending to mass produce her spinal implant. James is revealed to have intentionally lured Felicity into breaking the firewall so that he can have secret access and covered tracks. He provides Black Siren with a device to stop Curtis' trackers. Slade calls Oliver and asks for his help.