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.
There's a break in at Queen Consolidated's lab facilities where an extraordinarily strong man seems to have not only broken down a large metal door but carried out a large industrial centrifuge. A young CSI from Central City, Barry Allen, arrives to help out and says they've had a similar case. Oliver is suspicious and asks Digg to look into his background. Felicity certainly has a keen interest. Moira Queen meanwhile is now home again but can't help feeling like a marked woman. Oliver thinks they should throw a party but it's not as successful as they had hoped. Thea's biological father tells Moira he wants Thea to know about him.
After an impossible break in at Queen Consolidated, CSI Barry Allen comes to Starling to help with the investigation, Sin asks Roy for help so Thea tags along.
- We open at a Queen Consolidated Applied Science warehouse. Someone breaks through an enormous door and takes out two guards.
Moira returns to work. Queen places her at the head of the board room table and Isabel hates the idea. Digs tells Queen there's been a break-in.
At the scene of the break-in from the opening Lance tells Digs he thinks at least three or four men committed the crime. The security camera only shows one person. Barry Allen from the Central City PD's CSI shows up and says based on crimes in his town he thinks it was just one guy. Allen thinks an industrial centrifuge was probably the only thing stolen.
We flash to the island and the foursome rushing to find the sub in an attempt to save Slade. Queen tells Shado about his connection to Sara and Laurel.
It turns out Allen was right about everything. A camera picks up footage of a man carrying the enormous centrifuge by himself. Felicity arranges for Allen to help them with the investigation. It also seems she has a thing for the guy.
Queen suggests to Moira they should throw a welcome back party for her.
Allen asks Felicity about the Vigilante. Allen has several ideas about the person and most of them are dead-on. Allen admits that his mother was murdered when he was very young and wishes someone had helped. The soil tracked into the warehouse has sugar.
Sin calls Roy to tell him her friend Max is missing. Thea is there and wants both of them to help.
Malcolm drops by to see Moira. He says Thea is all he has left and demands she tell her the truth, "soon."
Felicity and Allen figure out a truck from the sugar refinery was stolen. They track the truck and see that it was used to steal blood from a blood bank. Digs thinks Queens isn't telling him everything.
We cut to Arrow following the truck with the stolen blood. He has little success fighting the made behind the wheel, but manages to stick an arrow in his thigh before being tossed from the moving vehicle.
Queen tells Felicity and Digs he last saw something like that man on the island. He tells them about Ivo and the serum that produced human weapons. He says Ivo and everybody from the island is dead, so somebody must be at it again. He wants Felicity to analyze the blood from the arrow. Digs has looked into Allen and "he's not who he says he is."
On the island Queen and the gang find the submarine.
Allen is excited the blood sample means the vigilante is working the same case they are. She asks him to go to Moira's party with her.
Sin tells Roy that Max went missing just prior to a recent blood drive organized by the Alderman. We see that a cop is watching Sin, Roy and Thea.
Queen confronts Allen in front of Felicity and asks him who he really is. It turns out his father has been serving a life sentence for killing his mother. Allen witnessed another person commit the crime but the police didn't believe his 11-year old version. "What I saw that night? Was real." He now looks into cases like this one in order to clear his father's name.
At the party Queen apologizes to Felicity for being so hard on Allen. She tells him he needs to apologize to his mother for the fact hardly anybody is at the gathering. Sin calls Roy and says she's found Max. Roy lies to Thea about the call so she doesn't join him. Queen surprises Felicity with word that he invited Allen. Isabel tells Queen she tried to warn him what would happen re Moira and the public.
Back on the island the gang goes inside the submarine. They finds the miracle serum and discuss giving it to Slade. Without any sedative to go with it, Sara thinks he'll die. They hear the sound of others entering the sub.
After the party Moira tells Thea "I love you," deciding not to drop the bomb about Malcolm.
Allen is getting pressure from his employer to return to work. He leaves
Roy arrives to find Sin next to Max's body. The police say he OD's but she doesn't buy it.
Based on the sedative in the thief's blood Felicity is able to track him. She warns Queen that his arrows won't stop a person with muscle density that thick.
On the sub Slade tells them to give him the shot. He's injected and starts to scream.
Roy calls Arrow and shows him the picture of Max with blood in his eyes. When Arrow tells him to stay away Roy says he dosen't need him anymore. Arrow shoots an arrow through Roy's leg.
Malcolm visits Moira. She drops the name Ra's al Ghul and says she told the League that Malcolm is still alive. They want to kill Malcolm themselves and she tells him he should "run."
Arrow fights the mysterious blood thief, attempting to extract information along the way. During the fight Arrow is thrown in to some boxes and a needle of some kind ends up embedded in his leg.
Back on the sub Slade screams in pain. Eventually he dies and blood pours from his eyes, much like what we saw with Max's body. At this point armed men storms into the room.
Digs and Felicity find Arrow unconscious. Felicity can't figure out what he was injected with. Digs thinks they should call 911 but Felicity has another idea.
Cut to Allen missing the last train out of town. He sits down and something hits him in the neck. He wakes up in Arrow's lair and sees Queen lying on a table. Felicity says to him "please save my friend."