With crime at an all time low, Oliver decides to take a break and asks Felicity on a date. However, they're interrupted when a new drug dealer take the name of Count Vertigo and targets the Arrow for death. Meanwhile, Diggle becomes an expectant father and a scientist tries to acquire Queen Consolidated before Oliver can retake his family's business.
Oliver and his team try to find the archer who killed Sara, and discover that a hitman calling himself Komodo had connections to their friends past with the League of Assassins. Meanwhile, Ray continues trying to hire Felicity, and Roy debates whether to tell Oliver why Thea left Starling City.
Felicity tracks Thea to the island of Corto Maltese, and Oliver, Roy, and Diggle got here to bring her back. Meanwhile, Lyla asks Diggle to look into the disappearance of ARGUS agent Mark Shaw, and Laurel looks for an outlet for her frustrations.
Nyssa believes that Malcolm killed Sara and goes after him to avenger her lover's murder. Oliver discovers that he has no choice but to stop her, despite Laurel insisting that he avenger her sister's death. Meanwhile, Thea returns to Starling City and settles in... until Nyssa discovers that she's Malcolm's daughter.
Felicity's past as a crusading hacker is revealed when someone uses a super virus that she designed to bring Starling City to a standstill. Meanwhile, Oliver and Thea work on their new relationship, and Felicity's mother pays a visit.
A killer is murdering his victims and attempting to frame Ted Grant. The Arrow finally comes to his aid after Laurel protests her trainer's innocence. Meanwhile, Roy goes to Felicity for help determining if he killed someone under the influence of Mirakuru.
The Arrow is stalked by an ex-police officer who he rescued during the Mirakuru siege and has become dangerously obsessed with the hero. Meanwhile, Ray asks Felicity to accompany him to a business dinner, and Thea is forced to hire a new DJ for Verdant.
Oliver, Arsenal, and Diggle track down the location of a boomerang-wielding killer named Digger Harkness but are surprised when they come face to face with an A.R.G.U.S. team. Diggle asks Lyla why A.R.G.U.S. is involved but she defers until Harkness attacks the building, killing several agents and targeting Lyla. The Arrow joins the fight and gets help from an old friend--the Flash. Harkness manages to get away and Oliver teams up with Barry again to find him before he can get to Lyla. When Harkness plants five bombs in the city that are timed to explode at the same time, both teams must come together to save the city.
Source: The CW