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.