Diggle and Felicity convince Oliver to return to Starling City despite the fact he blames himself for Tommy's death. He only returns to save Queen Consolidated, but soon becomes embroiled in a crisis when several men using the name of The Hood start killing the rich to get revenge for the Undertaking... and Thea Queen is their next target. Meanwhile, in the past on the island, Oliver, Slade, and Shado kill Fyers and his men, but soon receive a visit from someone new.
When criminals hijack medical supplies meant for The Graves, Oliver decides that the Hood should put a stop to the robberies. However, China White is responsible and has a new partner: the Bronze Tiger. Meanwhile, alderman Sebastian Blood continues to try to rally the city to help those in The Glades and considers an alliance with Oliver.
The killer known as the Dollmaker escapes from prison and goes back on another killing spree, targeting attractive women and choking them with liquid latex. Arrow and Quentin join forces to stop him using a willing Felicity as bait. When their plan fails, the Dollmaker goes after the daughter of the man who put him away. Meanwhile, DA Donner asks for the death penalty in Moira's murder case, and the Arrow seeks out Roy for help.
A new crime lord, The Mayor, plans to take over Starling City using stolen military weapons. While the Hood tries to stop him, he encounters Black Canary and discovers who she really is. When The Mayor injures Black Canary's sidekick Sin, the female vigilante joins forces with the Hood to take the gunrunner down. Meanwhile, Quentin confronts Diana about her drinking problem, and in the past, Oliver is interrogated about the corpses on the island.
Oliver puts Sara Lance up at the manor and soon discovers that she doesn't want to contact her family... both because she would put them in danger, and because she become a killer since the last time they saw her. Meanwhile, the League of Assassins send men to bring Sara back, alive or dead. And Oliver discovers that Laurel will be sitting second chair for his mother's trial.
A.R.G.U.S. abducts Diggle and asks him to help recover Lyla from a Russian prison. Oliver and Felicity insist on going along to help and Isabel tags along. Meanwhile, Jean Loring advises Thea that her dating Roy is hurting Moira's chance of getting an acquittal.
The Count has a new scheme to turn Starling City's residents into clients by addicting them to a new drug and then selling them Vertigo to cure it. among those stricken are Diggle and DA Donner, who collapses in the middle of presenting his case against Moira.
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 covering from a near-fatal drug overdose, Oliver begins hallucinating people from his past. Believing he's a liability, Oliver gives the information he has on Cyrus Gold to Quentin... which results in the deaths of several police officers. Meanwhile, Brother Blood captures Roy and uses him as a test subject for his new serum. In the past on the island, Ivo gives Oliver an impossible choice to make.
A bomber nicknamed Shrapnel has set his sights on Sebastian Blood, and Oliver dons his mask to protect his newfound ally. Meanwhile, Laurel investigates Sebastian's aunt who is locked up in a mental institution, and Thea learns about Roy's newfound superstrength.
When her father won't believe her accusations against Sebastian, Laurel turns to the Arrow for help. Oliver has his own doubts about Laurel, particularly when she is arrested on a charge of drug possession. Meanwhile, Roy sets out to test his new abilities but goes berserk, putting a man in the hospital.
While the Arrow trains Roy how to control his anger--and his super-strength--Bronze Tiger steals a prototype of Malcolm's earthquake machine to sell to an arms dealer. Meanwhile, Laurel discovers that she's being disbarred and goes out of control, and Walter has an offer for Moira. On the island, Oliver wonders whether to tell Slade the truth about how Shado died.
Sara returns to Starling City at Oliver's request to check on Laurel, and both he and Quentin would like her to stay. her situation becomes precarious when a member of the League of Assassins arrives in town and sets out to persuade Sara to fulfill her vow and return to the organization... no matter what it takes. Meanwhile, Felicity discovers Moira's secret and must choose whether to tell Oliver or not.
A master hacker dubbed Clock King coordinates a series of raids on banks in Starling City, and his skills prove a match for Felicity. Meanwhile, Laurel's grudge with Sara continues and on the island, a pilot crashes his plane there and Sara tends to the dying man.
In the present, Slade visits Oliver and makes his intentions clear. Five years earlier, Slade, Sara, and Oliver launch their assault on the Amazo, but things go wrong when Slade learns that Oliver was responsible for Shado's death.
Following Slade's return and his promise to make good on his threat to destroy Oliver's loved ones, Oliver focuses all his energy on preparing for battle with his former friend. Sarah tries to help but, fearing for her safety, Oliver pushes her away. Meanwhile, Diggle is recruited by Amanda Waller and A.R.G.U.S. to stop a warlord he knew from his time in Afghanistan. Always the solider, Diggle agrees to help. However, he's not prepared when Amanda introduces him to the rest of the team - Deadshot, Bronze Tiger, and Shrapnel, all criminals he and Arrow helped put away. Lyla defends the team and explains the world is a complicated place, but Diggle isn't sure he agrees. When the mission takes a dangerous turn, Diggle is forced to make a difficult decision.