After a seemingly impossible robbery at Queen Consolidated, Central City’s police scientist Barry Allen (Grant Gustin) offers to help Oliver and his team.
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 Count Vertigo (recurring guest star SETH GABEL) has broken out of prison and is once again distributing his drug. Meanwhile, Moira goes on trial for her participation in the Undertaking. When ADA Adam Donner collapses in court with Vertigo symptoms, Laurel must step in and try the case against Moira — leaving Oliver torn between standing by his family during the trial or catching Count Vertigo. The stakes change, however, when Felicity follows a lead and walks straight into a trap set by Count Vertigo.
Amanda Waller (guest star CYNTHIA ADDAI-ROBINSON) sends A.R.G.U.S. agents to kidnap Diggle. Waller informs Diggle that Lyla (guest star AUDREY MARIE ANDERSON) has gone missing after following a lead on Deadshot (recurring guest star MICHAEL ROWE) 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 (guest star TERYL ROTHERY), tells Thea that dating Roy, a known criminal, is hurting her mother's case.
The Canary sings! While Oliver tends to the Canary's wounds, he tries to get her to talk about where she came from, but she refuses. But when Oliver and the Canary are attacked at the Queen mansion by a trained killer from the League of Assassins, she confesses to how she's connected with the nefarious group. Meanwhile, with Moira facing multiple counts of murder 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.
Oliver meets the Canary. Oliver discovers a man who goes by the name "The Mayor" (guest star Cle Bennett) 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 (guest star Bex Taylor-Klaus). 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's identity, Donner asks Laurel to dinner, and Diggle reconnects with an old friend.
Officer Lance's past puts Laurel in danger. Lance learns a criminal named Barton Mathis (guest star Michael Eklund), 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 murder her while Lance watches. Meanwhile, Thea and Oliver are stunned when District Attorney Adam Donner (guest star Dylan Bruce) seeks the death penalty for Moira. Oliver asks Roy to help him find the Canary (guest star Caity Lotz).
DC Comics' Bronze Tiger and China White wreak havoc on the Glades. Oliver finds out that medicine being sent to Glades Memorial Hospital is being hijacked by thieves – and without an influx of supplies, the hospital will close. When Roy attempts to stop the thieves, he fails and is arrested by Officer Lance. Oliver gets into a heated battle with China White (recurring guest star Kelly Hu) and her new partner, Ben Turner, aka the Bronze Tiger (guest star Michael Jai White), over the supplies. Meanwhile, Laurel makes plans to catch the Arrow, and Thea gives Roy an ultimatum.
In the season premiere, Oliver must transform from vigilante to hero. Diggle and Felicity travel to the island of Lian Yu in search of Oliver, who left Starling City after the devastation of The Undertaking and his failure to avert it. Convinced to return home, Oliver finds that things are in much worse shape than he imagined – Isabel Rochev (recurring guest star Summer Glau) 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 Oliver's mentors Slade (new series star Manu Bennett), Shado (guest star Celina Jade) and Oliver in a desperate situation.
Watch Arrow and Diggle sparring to prepare.
Heroes are forged and sacrifices made in this epic season finale as Oliver and Diggle race to stop the Dark Archer from unleashing his vengeance on The Glades. However, they run into a roadblock after Detective Lance picks up Felicity for questioning. Tommy and Oliver's already tumultuous relationship takes a turn for the worse after Oliver makes a confession about Laurel. After hearing of the danger in The Glades, Thea races to find Roy, inadvertently putting herself directly in the line of fire for Malcolm's devious plan. On the island, Oliver, Slade and Shado are locked in a life-or-death struggle against Edward Fyers (recurring guest star SEBASTIAN DUNN) as his missiles lock on a full Ferris Air jetliner.
With the Undertaking days away, Malcolm dons his black hood to conclude business with seismologist Dr. Brion Markov (guest star ERIC FLOYD) and his team. Oliver and Moira entertain a number of surprise – and mostly unwelcome – visitors. Hooded confrontations at home and the Meryln Global Group reveal dark secrets. Meanwhile, Roy and Thea are still looking out for a hero. Laurel makes a decision after Tommy's revelation. In a flashback to the island, Oliver, Slade and Shado struggle to prevent Fyers (guest star SEBASTIAN DUNN) from shooting a Ferris Air jet out of the sky.
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 (guest star JAMEY SHERIDAN) and Frank Chen (guest star CHIN HAN) painful details surrounding his wife's murder, and how that shaped his plans for the Glades. Meanwhile, Tommy stuns Laurel with the truth.
Deadshot returns to Starling City after taking down a U.S. Ambassador and six others overseas. By keeping tabs on A.R.G.U.S.'s manhunt for the assassin, Oliver, Diggle, and Felicity learn that a sting operation is underway to capture Deadshot – alive. Diggle is determined to see his brother's killer dead, with or without Ollie's help. Meanwhile, Laurel takes drastic steps to protect a young witness, exacerbating friction between Tommy and Oliver. Later, Roy Harper unconventionally pursues all leads to find his new hero, but runs afoul of Detective Lance. Back on the island, Oliver recalls how Shado helped him incrementally gain confidence with a bow and hit the target for the first time.
When a young woman meets a violent death after partying at Verdant, Detective Lance and his partner, Detective Hilton (guest star ROGER CROSS) inform Oliver and Tommy that she was on the drug Vertigo. Oliver immediately pays a visit to the mental institution where The Count (guest star SETH GABEL) is locked up, but finds he is in no condition to deal drugs. However, when The Count escapes and Starling City is flooded with even more Vertigo-related violence, Oliver and Diggle make it their mission to track him down. After Detective Lance uncovers incriminating evidence against him, Tommy goes to great lengths to clear his name, and the fallout from his actions leads Tommy down a new, unexpected path. In a flashback to the island, Oliver recalls the lessons he learned from Slade and Shado (guest star CELINA JADE).
An angry resident of The Glades who calls himself "The Savior" (guest star CHRISTOPHER REDMAN) embarks on a kidnapping and killing spree to punish those he believes have wronged the residents of his neighborhood. To make things worse, he broadcasts these murders online and claims he was inspired by Arrow's vigilantism. Oliver is horrified by this statement and vows to stop "The Savior," but his manhunt in intensified after Thea comes to him in tears and tells him Roy has been kidnapped. Laurel is surprised when her father starts to believe her mother's claim that Sara is still alive. Meanwhile, Malcolm tells Moira to find the person responsible for his attempted murder.
With his nightclub set to open and a new romance brewing with McKenna, Oliver is as happy as he's been since returning to Starling City. But when Oliver's ex, Helena Bertinelli aka The Huntress, suddenly returns to town, she threatens to destroy everything and everyone he cares about. Meanwhile, Laurel and her father struggle with the return of Laurel's mother, Dinah Lance (guest star ALEX KINGSTON), who claims she has proof Sara is still alive. Thea gets Roy Harper a job at Oliver's club, but he's not too inclined to stop his life of crime. Oliver and Tommy are thrilled they were able to get EDM megastar STEVE AOKI (guest starring as Himself) to play at the opening of their new nightclub, Verdant.