Oliver, Arsenal and Diggle track down the location of a boomerang-wielding killer named Digger Harkness. 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.

Barry is thrilled when Oliver, Felicity and Diggle come to Central City to investigate a case involving a deadly boomerang. Barry asks Oliver if he'd like to help him stop Roy Bivolo, the meta-human Barry is currently tracking. Bivolo causes people to lose control of their emotions and has been using that skill to rob banks. Unfortunately, the superhero partnership doesn't go as smoothly as Barry expected. Bivolo infects Barry and sets him on a rage rampage, everyone is in danger, and the only one who can stop him is the Arrow.