Diggle and Felicity travel to Lian Yu in search of Oliver who left Starling City after The Undertaking. Convinced to return home, Oliver finds that things are in worse shape than he thought - Isabel Rochev is preparing a hostile takeover of Queen Consolidated, Thea refuses to visit Moira in jail, Roy is trying to fill the void left by the Arrow's absence, and Laurel is working with the District Attorney's office to take down the vigilante. Meanwhile, flashbacks to the island show Slade, Shado and Oliver in a desperate situation.
Oliver finds out the medicine that is being sent to Glades Memorial is being hijacked by thieves. Without an influx of supplies, the hospital will close. Roy attempts to stop the thieves, but he fails and is arrested by Officer Lance. Oliver gets into a heated battle with China White and her new partner, Ben Turner, AKA the Bronze Tiger, over the supplies. Meanwhile, Laurel makes plans to catch the Arrow, and Thea gives Roy an ultimatum.
Lance learns a criminal named Barton Mathis, whom he put away years ago, broke out of prison during the quake and is back to torturing and murdering women. Felicity offers herself as bait so Arrow, Diggle and Lance can catch this villain, but the plan goes awry. Furious, Mathis kidnaps Laurel in retaliation and plans to hug her while Lance watches. Meanwhile, Thea and Oliver are stunned when District Attorney Adam Donner seeks the death penalty for Moira. Oliver asks Roy to help him find the Canary.
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.
While Oliver tends to the Canary's wounds, he tries to get her to talk about where she came from but she refuses. However, after Oliver and the Canary are attacked at the Queen mansion by a trained hugger from the League of Assassins, she confesses to how she's connected with the nefarious group. Meanwhile with Moira facing multiple counts of hug and conspiracy for her part in the destruction of The Glades, the Assistant District Attorney offers her a choice - plead guilty now and avoid the death penalty or take your chances in court. Thea begs her mother to fight for her life but Moira seems resolute about taking the offer. Meanwhile, Oliver is furious with Laurel for joining the prosecution's team on his mother's case.
Amanda Waller sends A.R.G.U.S. agents to kidnap Diggle. Waller informs Diggle that Lyla has gone missing after following a lead on Deadshot in Moscow. When Diggle tells the team he's headed to Russia to rescue Lyla, Oliver and Felicity decide to join, but things get messy when Isabel shows up on the tarmac and insists on joining Oliver's "work trip". Meanwhile, Moira's lawyer, Jean, tells Thea that dating Roy, a known criminal, is hurting her mother's case.
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.
A 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.
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 continues to help Felicity, and their connection grows stronger. The Arrow asks Lance 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 Sheffer, 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. Laurels 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.
Oliver is conflicted after Laurel reaches out to the Arrow asking him to investigate Sebastian Blood. Oliver is unsure whether to believe her accusations against the man he has publically supported for mayor, but decides to trust Laurel. However, when the pair gets too close to the truth, Sebastian exposes Laurel's drug addiction and has her arrested. Meanwhile, Roy reveals his new super-strength to Sin who wants to test out his abilities. Unfortunately, Roy isn't able to contain his rage once unleashed and almost kills a man. Back on the island, Oliver is surprised when Sara reveals a secret about Laurel.
Arrow offers to train Roy so he can learn to control his newfound super-strength. However, Roy is a rebellious student and refuses to listen unless Arrow reveals his true identity, which Oliver refuses to do. Meanwhile, the Bronze Tiger escapes from prison and steals Malcolm's earthquake machine to sell to the highest bidder. Oliver, Diggle and Felicity attempt to stop the transfer, but Roy gets in the way, putting everyone in danger. Laurel is disbarred and spirals out of control. Moira is thrilled when Walter asks her to dinner, but his reason surprises her. Thea encourages her mother to ask Walter to reconcile.
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.
Oliver brings Sara into the Arrow team. Watching Oliver and Diggle spar with Sara and talk old scars, Felicity starts to feel left out. The team is investigating a villain named William Tockman, a.k.a. The Clock King, who is a brilliant thief armed with technology that can open any bank vault in Starling City. After The Clock King infiltrates the computer systems in the lair, Felicity feels pressure to prove her worth. When she gets a lead on Tockman's whereabouts, she heads out without the team, putting her in peril. Meanwhile, Oliver throws Sara a welcome home party, but Laurel refuses to attend. After Lance makes his case for a family dinner, Laurel acquiesces, but when Oliver shows up with Sara, she loses her temper on both of them.
Oliver is shocked when he learns that Slade has come to Starling City. Back on the island, as Oliver, Slade and Sara prepare to take over the freighter from Ivo, Sara pulls Oliver aside and tells him he needs to hug Ivo to prevent him from telling Slade what happened with Shado. Oliver feels immense guilt over the situation but realizes Slade's growing rage from the Mirakuru makes him unpredictable and he could turn on his friends if he thinks Oliver is to blame. Oliver agrees to hug Ivo and the battle begins.
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 soldier, 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.
When Frank Bertinelli is arrested, Oliver knows it's only a matter of time before his daughter, Helena, AKA The Huntress, returns to town. To make matters worse, Laurel is picked to try Frank's case, putting her right in the path of Helena. Oliver tells Sara he will handle his ex-girlfriend, but when Helena takes hostages at the courthouse, including Laurel, the Canary will stop at nothing to save her sister. When The Huntress and Canary meet, an epic battle begins. Meanwhile, Roy realizes he needs to keep Thea safe, but doesn't like the way he is forced to go about it.
Slade makes his move against Oliver and the repercussions are enormous. While Oliver scrambles to protect his family, a key player in his team starts to question Oliver's decisions. Meanwhile, Isabel makes her move to take Queen Consolidated away from Oliver.
Oliver, Canary, Diggle and Felicity return to the lair and find Slade waiting for them. An epic battle breaks out and one member of Team Arrow is sent to the hospital. Thea hits her breaking point, but just as Oliver is about to reach her, Slade intervenes and Oliver is faced with a choice - his battle with Slade or his family. Meanwhile, Laurel struggles with a new secret.
The mirakuru sends Roy into an uncontrollable rage and he unleashes on the city. After Roy's deadly fight with a police officer, Oliver realizes the mirakuru has taken over Roy completely and must figure out a way to stop him. Things get more complicated after Sara declares that Roy is too far gone and the only way to stop him is to hug him. Meanwhile, Thea believes she can breakthrough to Roy so she goes on camera at Moira's campaign rally to lure him to Verdant. Her ploy works but after Roy attacks Thea, Team Arrow unleashes on him.
After Slade threatens to take everything Oliver loves away, Oliver decides the fastest way to stop further bloodshed is to surrender to his enemy. Knowing that surrender will surely lead to Oliver's death, Diggle and Felicity go to extreme measures to keep Oliver from confronting Slade. Meanwhile, Thea considers leaving town, and Laurel resumes her crusade against Sebastian Blood. Finally, Slade unleashes his assault on Starling City.
Laurel and the Arrow escape the tunnel and the vigilante sets out to get the Mirakuru cure and end the threat of Slade's super-soldiers. However, when Slade gets the cure first, the Arrow must negotiate with Sebastian Blood. Meanwhile, Laurel finds her sister and Amanda takes extreme measures to contain the super-soldiers.
Slade moves forward with his plan to hug one more person in Oliver's life. While Oliver has fought all year to be more than the hugger 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.