After Detective Lance arrests Oliver for murder, Oliver tells Moira the only lawyer he'll allow to represent him is Laurel. Laurel takes the case which puts her at odds with her father. Oliver offers to take a polygraph in front Detective Lance but things get tense when Lance asks him if anyone else was on the island with him. Oliver flashes back to when he got his first scar, courtesy of Deathstroke.

Finding it difficult to mend fences with Tommy and Diggle, Oliver instead focuses on crossing another name off the list. While digging through a crooked accountant's laptop, Felicity discovers a transaction that could help Oliver find Walter. To reconfirm the lead, Oliver gambles that Felicity can be counted on in the field for the first time. In a flashback, Malcolm reveals to Robert Queen and Frank Chen painful details surrounding his wife's murder, and how that shaped his plans for the Glades. Meanwhile, Tommy stuns Laurel with the truth.

When a superhumanly strong thief robs a Queen Consolidated warehouse, Central City police scientist Barry Allen comes to Starling City to investigate the case. However, despite his close working relationship with Felicity, Oliver suspects that there's more to the young scientist than meets the eye. Meanwhile, Sin asks Roy to help investigate the disappearance of a friend. In the past, Oliver, Shado, and Sara try to get Slade to the submarine so they can use the serum to save his life.

After the death of someone close to him, Oliver decides to give himself up to Slade rather than risk further bloodshed on his friends and family. Meanwhile, Laurel renews her investigation into Sebastian Blood, and Slade makes his final plans to unleash his Mirakuru-enhanced army on Starling City.

Felicity's past as a crusading hacker is revealed when someone uses a super virus that she designed to bring Starling City to a standstill. Meanwhile, Oliver and Thea work on their new relationship, and Felicity's mother pays a visit.

Vertigo causes Laurel to hallucinate an epic fight that pits her Black Canary against her sister Sara. Meanwhile, Chase surprises Thea and Roy warns Thea to stay away from Malcolm.

Brie Larvan breaks out of jail and turns up at Palmer Tech threatening to kill everyone unless she gets the bio-chip that is implanted in Felicity’s spine. When Oliver realizes that Felicity, Thea and Donna are trapped in Palmer Tech with everyone else, he looks to a surprising ally to help Team Arrow save the day.

Oliver faces his biggest challenge yet as mayor. Felicity continues down her dark path with Helix. After Vigilante attacks Oliver while he's acting as the mayor, Diggle leads the team in a mission to stop Vigilante once and for all.