The next day, Moira Queen returns to work, having just recently been acquitted on 503 counts of murder. Felicity is there to welcome her back, but in true Felicity fashion, winds up sticking her foot in her mouth. Moira and Oliver then join a meeting already in progress, only for Oliver to get pulled aside by Isabel Rochev. Isabel questions the wisdom in handing the company right back to Moira, pointing out that the Queen matriarch may have been acquitted in a court of law but certainly not in the court of public opinion. She tells Oliver to stop thinking like a son and start thinking like a CEO. Just then, Dig and Felicity inform Oliver that there's been break-in. At the scene of the crime, Quentin is taking Oliver and his team through what happened, theorizing that a team of three or four guys were behind it, when Barry Allen, a young CSI from Central City, arrives, insisting that only one, albeit really strong, person was behind it. He then introduces himself, apologizing profusely for being late, even though no one was apparently expecting him. He says he's investigating a case with "similar unexplained elements," and that he was sent there by his captain. Barry goes on to deduce that an industrial centrifuge was stolen, pointing out that the thief ripped it right out of the concrete. Taking Felicity aside, Quentin tells her she may want to tip off their mutual friend, the Arrow, but Felicity cryptically replies that he's already on it. Meanwhile, in an island flashback, Oliver, Sara, Slade, and Shado are making their way to a Japanese sub, hoping to find a miracle serum that can heal Slade. As they all stop to rest a moment, Shado asks Oliver how he and Sara know each other, leaving him to awkwardly explain the circumstances of him and Sara being stranded on the island (i.e. he was cheating on his girlfriend Laurel with her sister). Back in the present, Barry offers his help keeping the investigation in-house, and Oliver reluctantly accepts. As Felicity goes to help Barry set up his equipment, Dig observes that Oliver seems to know more about the case than he's letting on, to which Oliver cryptically replies, "Pray I'm wrong."
Later, Oliver finds Moira back at the mansion. He apologizes for the awkwardness of that morning, and offers to throw her a party to help get things back to normal. She's reluctant at first, but Thea appears with Roy just then and talks her into it. Meanwhile, at the Applied Sciences warehouse, Barry asks Felicity about the Arrow, theorizing that the Arrow has trained in a forest or jungle environment (hence the green) and that the vigilante has partners, including someone with a computer sciences background. He explains that the reason why he's so fascinated with the Arrow is because his mom was murdered when he was a kid and that they never caught the killer, figuring that maybe the Arrow would've been able to do so. That night, Roy and Thea meet with Sara's friend Sin in the back alley of Verdant. Sin says that her friend Max has been missing for a week, and figures Roy knows someone who could help. Rather than letting the Arrow handle it, though, Thea volunteers Roy to dig into Max's disappearance, and insists on coming along. Meanwhile, back at the Queen mansion, Malcolm Merlyn pays Moira a visit. He says that after the night of Thea's conception, he felt so bad about betraying his late wife Rebecca's memory that he left for Nanda Parbat and there, found a new purpose in life. He adds that with Tommy dead, Thea is all he has left, and demands that Moira tell Thea the truth about her father and tell her soon. In the meantime, back at the Applied Sciences warehouse, Barry and Felicity share some new developments with Oliver and Dig: the thief had tracked in sugar that had come from a refinery two miles away, and that that same refinery had had a delivery truck stolen, which the thief had used during the break-in. Just then, Felicity gets an alert that that same stolen truck had just been used to rob a blood bank. Oliver then tries to ask about the similar case Barry had been working on, but Barry is vague and evasive in his answer. So, taking Dig aside, Oliver asks him to look into Barry, feeling there's more to the CSI than what Barry's letting on. Oliver then goes after the thief as the Arrow, only to get tossed around like a rag doll. During the fight, Oliver stabs the mysterious thief with an arrow to the thigh, only for the arrow to have very little effect.
Later, at the lair, Oliver reveals to his team that he's come across men with abilities like that before. He explains that back on the island, he came across a Japanese serum from World War II that gave those who survived it enhanced strength, endurance, and reflexes. However, he insists that those who were injected with it and the doctor who'd experimented on them (Ivo) were all dead, and that he'd destroyed every last bit of the serum. Reasoning that someone has found a recipe to make more, though, Oliver informs them that there's one final ingredient that the thief will be after: a strong sedative. Handing his bent arrow to Felicity, he asks her to analyze the sedative in the thief's blood so they can figure out where the next robbery's going to be. While Felicity goes to do that, Dig apprises Oliver of his findings on Barry, revealing that Oliver was right and Barry is not who he says he is. In an island flashback, meanwhile, Oliver and friends find the submarine, which is sticking out of a lagoon in the middle of the island. Back in the present, Felicity brings the bloodstained arrow to Barry. While Barry's gadgets are doing their thing, she brings up the party Oliver's throwing for Moira, and asks Barry to be her plus-one. He accepts, though warns that he's not that great at dancing. The next day, Sin, Roy, and Thea comb the apartment of Sin's friend Max, and find a pamphlet for a blood drive run by Sebastian Blood that had taken place the day Max had disappeared. Unfortunately, as they're leaving, a cop named Daily spots them and reports their snooping to Sebastian Blood, who orders Daily to let them find Max. Afterwards, Blood pontificates to the mysterious thief, Cyrus Gold aka The Acolyte, that the stolen blood will give life and power to his disciples, and with that power, no one will be able to stop him. Meanwhile, Barry and Felicity are at Queen Consolidated, watching a report on a particle accelerator in Central City due to be turned on the next day, when Oliver arrives to confront Barry. Apparently, Barry is only an assistant CSI, and his boss had no idea he was even there in Starling City. Oliver also reveals that while Barry's mom was indeed murdered, Barry's dad is serving time for the murder. Barry insists that the police only think they've caught the killer, and that nobody had believed him when, at 11 years old, he'd told them a blur had attacked his mother and that in that blur he'd seen a person. He adds that ever since then, he's tried to make sense of strange cases like the one-man break-in in the hopes that eventually, he'll be able to figure out who killed his mom and exonerate his dad. Then, his tail between his legs, Barry tells Felicity to find another plus-one. Oliver tries to defend his actions to Felicity, pointing out that Barry did lie about who he really is. Felicity, though, retorts that they do the same thing every day.
That night, at the Queen mansion, Oliver heads downstairs with his mother and sister, all of them dressed for the party. Felicity is waiting at the bottom of the stairs, and Oliver tries to apologize for overreacting regarding Barry. However, she tells him it's his mother he should apologize to, and that's when Oliver and his family see that hardly anybody bothered to show up for the party. Ever the gracious hostess, though, Moira insists on mingling with what few guests they have. Among the guests is Roy, who gets a call from Sin, saying she found Max, though she doesn't sound happy about it. Roy excuses himself to meet up with her, assuring Thea that everything's fine. Over on the dance floor, Oliver asks if Felicity wants to dance, but she replies that she doesn't feel like dancing with him. Anticipating this, he directs her attention to the doorway, where Barry is standing, dressed in a tux. Leaving the lovebirds alone, Oliver then heads over to the bar, where he shares a moment with Isabel Rochev. Seeing this, Moira comes over to intervene, all but warning Isabel to stay away from her son. In a flashback, meanwhile, Oliver and company board the Japanese submarine, and immediately begin searching for the Mirakuru serum. While they do manage to find it, Sara insists Slade would need a sedative first; otherwise, he'll die. Unfortunately, there are no sedatives to be found, so Shado decides to give it to him anyway, figuring Slade's prognosis is grim either way. Just then, they hear Ivo and his men walking around above them, having apparently caught up to them.
Back in the present, Moira is having a conversation with some associate of hers, the party having already ended. She asks if he "made contact," and the associate replies that "he" greatly appreciated Moira's information. Moira then dismisses the associate, just as Oliver and Thea enter. Oliver apologizes for the party, but Moira says all that matters to her is what he and Thea think of her. Just then, Oliver gets a call from Felicity, and excuses himself, claiming he has to get back to the office. Back at the Applied Sciences division, Barry gets off the phone with his captain, who now knows Barry does not in fact have food poisoning. Gathering up his things, Barry informs Felicity that the sedative in the thief's blood was ketamine, before preparing to return to Central City to salvage his job. Meanwhile, Roy meets with Sin, only to discover that the police have cordoned off the area around Max's body. Sin tells Roy that the police think Max died of an overdose, but she insists he wasn't an addict, even pointing to the fact he made extra cash donating blood. So, Roy uses his phone to snap a picture of Max's face, but Officer Daily sees him and shooes him from the scene. Back at the lair, meanwhile, Felicity relays Barry's findings, informing Oliver and Dig that the only place with a large enough supply of ketamine is an A.R.G.U.S. disaster bunker on the edge of the Glades. Just as Oliver's getting ready to suit up, though, Roy's red flechette is spotted in the back alley of Verdant. Meanwhile, in an island flashback, Oliver presents the serum to Slade, as well as the information that the serum could either save or kill him. Fearing he might die anyway, Slade decides to take it, but not before sharing a moment with Shado, wherein he apologizes for not telling her how he really feels. He then lets Oliver inject him with the serum, making him scream and convulse with pain. Back in the present, Oliver meets with Roy as the Arrow. Roy tells him about Max, even showing him a picture of blood streaming from Max's eyes. Recognizing the symptoms, Oliver warns Roy to stay away from that, an order which does not sit well with Roy. Instead, Roy decides to continue investigating Max's death with Thea and Sin, and Oliver responds by shooting him with an arrow to the knee, hoping it'll slow Roy down enough for Oliver to take down the thief and whoever's behind the making of the serum.
Back at the mansion, meanwhile, Malcolm pays Moira another visit. This time, though, she tells him in no uncertain terms that he is not to go anywhere near Thea. Apparently, after he mentioned Nanda Parbat, she did some digging, and reached out to Ra's al Ghul, who was very grateful to learn that Malcolm was still alive as he wants very much to kill Malcolm himself. Temporarily defeated, Malcolm leaves, but not before declaring, "This isn't finished." Meanwhile, Oliver/The Arrow finds the thief at the A.R.G.U.S. bunker. Oliver tries to question him, but doesn't get much information out of the thief. Instead, they fight, with Oliver landing among a pile of syringes, two of which are sticking out of his leg. Back on the island, meanwhile, Slade's eyes start bleeding from the serum, before finally, he dies. Unfortunately, before Oliver and Shado can even begin to mourn him, Ivo and his men burst in. Back in the present, Dig and Felicity find Oliver passed out in the bunker, the thief already gone. Felicity tries to figure out what it was Oliver was injected with, but unfortunately, the labels on the syringes are coded and she can't break into the dispensary files. Worried, Dig starts to call 9-1-1, but Felicity stops him, as she has another idea. Meanwhile, Barry arrives at the train station, to find the last train to Central City has already left and the next one won't leave until in the morning. So, he has a seat, intending to wait until then, when he's suddenly hit with a tranq dart. When he wakes up, he finds himself in the lair, Felicity standing over him and Oliver passed out on a table. Felicity pleads with him, "Please save my friend."
