- "To fight the unthinkable, you have to be willing to do the unthinkable."
- —Sara Lance to Oliver Queen about her decision.
Slade moves forward with his plan to kill one more person in Oliver's life. While Oliver has fought all year to be more than the killer he once was, when Slade kidnaps someone close to Oliver's heart, Oliver is pushed to the edge and realizes sometimes it takes doing the unthinkable to stop the monster. Meanwhile, Diggle takes on Amanda Waller with a little help from some friends, and Thea turns to Roy in her time of need.
- Stephen Amell as Oliver Queen/The Arrow
- Katie Cassidy as Laurel Lance
- David Ramsey as John Diggle
- Willa Holland as Thea Queen
- Emily Bett Rickards as Felicity Smoak
- Colton Haynes as Roy Harper
- Manu Bennett as Slade Wilson/Deathstroke
- Susanna Thompson as Moira Queen (credit only)
- Paul Blackthorne as Quentin Lance
- Caity Lotz as Sara Lance/The Canary
- Celina Jade as Shado
- Summer Glau as Isabel Rochev/Ravager
- Audrey Marie Anderson as Lyla Michaels
- Cynthia Addai-Robinson as Amanda Waller
- Katrina Law as Nyssa al Ghul
- Michael Rowe as Floyd Lawton/Deadshot
- John Barrowman as Malcolm Merlyn/Dark Archer
- Viv Leacock as Raven
- Keri Adams as Bethany Snow
- Michael Adamthwaite as Torque
- Jennifer Cheon as Uni
- Thai-Hoa Le as Man in Suit
- Terry Lewis as Terrified Prisoner
- Eric Mazimpaka as A.R.G.U.S. Guard
- David Nykl as Anatoly Knyazev
- Allison Riley as A.R.G.U.S. Tech
Oliver, Felicity and John are waiting for Roy to be cured. Oliver gives up, saying the cure clearly doesn't work. Before Roy could wake up, Slade's men show up, setting off alarms Felicity set. However, Roy wakes up and fights with Oliver in an ambush by Slade's Mirakuru army.
After leaving the clocktower, Diggle's girlfriend, Lyla Michaels, blows up the clocktower, killing the Mirakuru-enhanced soldiers.
Roy, Felicity, Oliver, Lyla, and Dig head to the foundry and find it destroyed. Oliver mentions how ARGUS was going to bomb the city. Oliver vows to not do any killing but shoot the cure.
Thea then shoots Malcolm twice but he survives because he was wearing a kevlar vest. Malcolm tells Thea that he is proud of her for shooting him. He then tells her that Tommy had held a gun against him also but lacked the courage to shoot him. He then admits to being proud of her.
Meanwhile, at the police headquarters, Quentin is busy rallying troops for the fight. He tells Laurel to stay safe and asks after Sara. She tells him she won't step out. Laurel finds Sara with a stranger who makes her unconscious.
Meanwhile, Team Arrow and Lyla are then joined by Sara and the League of Assassins led by Nyssa. Sara then tells Oliver that she agreed to go back to the League in return for their help with fighting Slade in the effort of keeping her family safe.
Roy asks after Thea and learns that she left town. Thea, then gets a call from Roy who is completely cured. Roy tells her that he wants to meet her at his house, to which she agrees. Malcolm then tells Thea that Roy is one of the liars who should not be trusted. He tells her that he will be there for her when she needs him the most.
Team Arrow and the League of Assassins go to Slade's headquarters where they fight but Slade manages to escape. Nyssa kills Isabel Rochev, as Isabel boasts about him losing.Nyssa then says that Oliver has to start killing again to win.
Quentin rushes to Laurel, as she wakes up and tells him about Nyssa. Meanwhile, one of Slade's followers attacks Quentin and kidnaps Laurel.
Quentin meets Team Arrow and tells Oliver that he respects the latter's non-killing promise. However, he tells Oliver that he should start killing again because Slade is a monster who needs to be put down for good. Quentin tells him that he needs to go get Laurel. Oliver, while talking to Felicity, agrees that Slade is a monster and needs to be killed. Felicity, however, tells him that Oliver made a promise which he has to keep.
Later, Thea meets Roy, who tells her that he has one last job to do, after which the two of them can start new lives together.
Meanwhile, Oliver tells Felicity to stay in his old house because she is the woman he loves most.
Amanda Waller orders a strike but can't reach a tunnel full of men that are all dead.
Team Arrow and the Assassins head to cure the army as Dig and Lyla let the Suicide Squad free and tell them to attack without killing due to the bombs implanted in their spines.
While Roy is gone, Thea begins to pack for Roy and finds Roy's red bow.
Later, the squad meet up with Amanda Waller who had ordered a drone strike on Starling City. She reveals that Lyla is pregnant.
Oliver then gets a call from Slade, Oliver told him he's sick of his games but he tells him that he has the person Oliver loves most - Felicity.
Oliver goes to meet Slade, who says that he keeps his promises. Slade then tells Oliver to decide between Felicity and Laurel. Oliver asks Slade about his hallucinations about Shado. He tells Slade that Shado was a very kind and brave lady who would not approve of the monster Slade has transformed into. While distracting Slade, Felicity injects the cure into Slade, ridding him of the Mirakuru.
Flashbacks reveal that Oliver brought Felicity to Queen Mansion purposely with a plan to say a fake "I love you" to lure Slade in, who had been watching the bugged mansion ever since his first visit. Flashbacks also reveal that Oliver discovered one of the bugs after his mother's murder.
Sara arrives and shoots the guy holding Laurel. Slade and Oliver then fight, with Slade telling Oliver that there is only one possible outcome to this. Oliver would have to kill him to win. He says that Oliver will remain a murderer.
On the Amazo, Oliver and Sara are captured by Slade who is injecting the Mirakuru in various prisoners with little success. Around the same time, Anatoly Knyazev apologizes to Oliver, and launches a torpedo, which stops Slade from shooting Oliver. Oliver then tells Sara to escape and fights Slade, however she gets sucked out of a hull breach and Oliver presumes her to have drowned. After a huge fight, Oliver emerges victorious as Slade is trapped by falling debris. Slade tells Oliver that he will come after everyone Oliver loves - his sister, Laurel, his mother, enraging Oliver who puts an arrow through Slade's eye, assuming him to be dead, instead of curing him.
In the end, Oliver uses one of his trick arrows to trap Slade. After trapping Slade, Oliver calls Amanda and tells her that he has Slade and she can stop the attack now. Amanda finally budges and stops the nuclear attack.
Sara meets up with her sister and father. He questions why she chose this, and asks Nyssa. Sara tells them that she chose this. She then hugs her father. Nyssa tells them that she would die before anything happened to Sara. Laurel and Sara then share a moment together. Sara tells Laurel that Oliver needs her before giving her the leather jacket, vowing to be back. Laurel says that she will be waiting for Sara's return. After Sara leaves, Laurel and Quentin are walking back when Quentin suddenly begins to stumble and cough up blood due to internal bleeding from his injury from the Mirakuru enhanced soldier. Laurel calls an ambulance to help.
Roy returns home looking for Thea and spots her note. "Roy I'm sorry. I made a mistake. Thought I could still be with you. Thought I could still be Thea Queen...But Thea Queen was trusting. And I don't trust you. Or anyone. Not anymore. Thea Queen was also weak. And no matter what it takes, I will be strong. Don't try to find me. You won't. Even I don't know where I'm going. I only know one thing...I am never coming back. Goodbye, Thea." The note read. Thea leaves Starling City with Malcolm in his limousine.
Slade opens his eye to see Oliver who thanks Slade for teaching him to kill when he most needed it. Slade tells Oliver that he will be back and see his promise completed. Oliver says goodbye to Slade, telling him that the latter is locked away from the rest of the world so as to not do any harm. "You're in purgatory," Oliver tells him.
Oliver then meets up with Diggle and Felicity, telling them that A.R.G.U.S' prison on Lian Yu is more secure than any other in the world. He mentions trying to get back his company but is broke and needs a job.
Diggle then leaves the two for a moment of privacy where Felicity praised Oliver's deception on Slade but confessed that for a moment she believed his feelings for her were real stating that he "really sold it". Oliver smiles then states "We both did". Then he asks Diggle about Lyla but Diggle diverts the topic, telling Oliver that it's not important. Oliver tells Diggle that he's flying back home. Felicity asks Oliver how he knows how to fly a plane if he was stranded on the island for 5 years.
It then flashes back to after the boat exploded, where Oliver wakes up in a bed in an unfamiliar room. He is given a shirt and told to follow two men with guns. He steps outside into the light and is greeted by Amanda Waller who tells him, "Welcome to Hong Kong, Mr. Queen."
- When leaving the island at the end of the episode, Diggle refers to Slade's new prison as "Argus's Super Max." Super Max was the name of a long-in-development script by David Goyer which would have seen Green Arrow attempting to escape from a prison made for supervillains after being placed there for a crime he didn't commit. The movie would've seen cameos from several DC villains as inmates of the prison.
- This is the only season finale that Oliver hasn't killed, or attempted to kill, the main antagonist of the season and the antagonist survives.
- This was the only season finale to feature Sara Lance until the Season 6 episode "".
- This is the final episode to credit Manu Bennett as a series regular as he is later downgraded to recurring for seasons 3, 5, and 6.
- This marks the final appearance of Susanna Thompson as a series regular.
- After this episode, Anatoly Knyazev would not be seen again until the Season 5 episode "".
- The final flashback confirms that Oliver didn't spend all of his five years away on Lian Yu.
- Preparation ran from March 28 until April 7, 2014. Shooting ran from April 8 until April 17, 2014.
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