Seemingly impossible robbery at Queen Consolidated's Applied Sciences Division brings Central City police scientist Barry Allen to town. Citing a similar case back home, Barry offers to help Oliver and team with the investigation. Oliver senses there is more to Barry than meets the eye, but he's distracted by the similarities between this current case and something that happened on the island. Meanwhile, Felicity takes a liking to Barry, which doesn't go unnoticed by Oliver. Sin asks Roy for help when a friend of hers goes missing. Roy is surprised when Thea not only encourages him to help, but joins the search. Unfortunately, Sin's friend is connected to Brother Blood, and their search ultimately gets one of them seriously injured.

Oliver must tackle a new threat to the city when bombs start to go off in Starling City. Felicity discovers the person setting them off is Mark Sheffner, a man who goes by the nickname Shrapnel. Arrow discovers Shrapnel's next target is Sebastian Blood's "Unity Rally" and tries to talk the alderman out of hosting it, but he refuses. Arrow sets off to stop the bombing, but Shrapnel tricks him and traps him in a bomb-laced antique store where he is unable to move without setting off the device. While Felicity helps Oliver, Diggle races to the plaza to find the bomb before it goes off. Meanwhile, Roy continues to hide his new strength from Thea but, after she witnesses his super strength in action at the rally, she demands answers. Laurel's suspicions about Sebastian grow stronger once she learns that he grew up with Cyrus Gold, the man who killed Lance's partner. After Donner refuses to help her investigate, she turns to Arrow for help.