Oliver discovers a man who goes by the name "The Mayor" is bringing illegal guns in to The Glades, so Oliver sponsors a "Cash for Guns" event to help clean up the city. Unfortunately, The Mayor crashes the event with his gang and seriously injures Sin. The Canary is furious when she learns her friend is in the hospital and sets out for revenge. Meanwhile, Felicity stuns Oliver with a bit of information about the Canary, Donner asks Laurel to dinner, and Diggle reconnects with an old friend.

Oliver is concerned when a mysterious illness sweeps the city, infecting hundreds of people, including Diggle. Oliver discovers Vertigo in Diggle's blood and realizes The Count has broken out of prison and is once again distributing his drug. Meanwhile, Moira goes on trial for her participation in the Undertaking. ADA Adam Donner collapses in court with Vertigo symptoms so Laurel must step in and try the case against Moira. Oliver is torn between standing by his family during the trial and catching The Count. However, things change when Felicity follows a lead and walks straight into a trap set by The Count.

After an impossible break in at Queen Consolidated, CSI Barry Allen comes to Starling to help with the investigation, Sin asks Roy for help so Thea tags along.

After a fight with Cyrus Gold, aka the Acolyte, Oliver is drugged and left for dead. Determined to cure him, Felicity makes a risky decision. The cure causes Oliver to hallucinate, and what he sees forces him to confront his past. Meanwhile, Barry Allen continues to help Felicity, and their connection grows stronger. The Arrow asks Quentin to look into Gold, but when the stakeout on Gold goes bad, someone is killed. Roy is captured and tortured by Brother Blood.

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 Scheffer, a man who goes by the nickname Shrapnel. The 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. The 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 The Arrow for help.

When Laurel asks the Arrow to investigate Sebastian Blood, Oliver is unsure whether to believe her accusations. But when they get too close to the truth, Blood strikes back.