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.

In the aftermath of Oliver's fight with Ra's al Ghul, Diggle and Arsenal continue to protect the city in the Arrow's absence. However, after three days without hearing from Oliver, they begin to fear the worst may have happened to their friend. Felicity refuses to believe that Oliver could be dead until Merlyn pays the team a surprise visit. Thea suspects there is something more behind Oliver's disappearance and asks Merlyn for a favor. Meanwhile, Ray tests a part of his new suit and Laurel takes up the mantle of the Black Canary.