- "I'm Barry Allen"
- —The CSI from Central City introducing himself
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 his 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.
When a masked thief with superhuman strength steals a centrifuge from Queen Consolidated's Applied Sciences Division, Oliver and his team investigate, with the help of Central City CSI Barry Allen, who is a fan of the Vigilante and has many observations and theories about him. He reveals all of this to Felicity, telling her he's interested because his mother was murdered and a person like The Arrow may have been able to stop it.
Oliver suspects that Barry is hiding something and has him investigated by Diggle. When confronted with the fact that there is no similar case in Central City and he's only an assistant CSI, Barry reveals that he is searching for super-powered beings in the hope that he will prove one case true and subsequently, find answers to clear his father for the his mother's murder. He explains the night his mother died, when a blur-like figure entered his home and his father went to fight it. Barry went to help but somehow found himself twenty blocks away. It's implied that the strangeness of his testimony and the fact that he was so far away led the police to believe he was covering for his father, who was arrested and convicted of his mother's murder.
After a blood bank is robbed and Oliver fails to stop the thief, he recognizes the combination of blood and serum and realizes that the thief was enhanced using the same serum that Dr. Ivo was searching for on the island. He reveals this information to Diggle (who had previously suspected he knew more than he was telling) and Felicity adding that he destroyed the entire supply of the serum and all survivors were subsequently killed. He relays to Diggle and Felicity that the serum needs a strong sedative as well, which leads Felicity (aided by Barry) to find the next place the robber will hit. Felicity warns Oliver that that he may not be able to fight the thief due to his strength and muscle density.
Roy and Thea stumble upon Oliver's case after helping Sin to find her missing friend. He was a test subject of Alderman Blood's that didn't survive after being lured in by a blood drive. He contacts Oliver to ask him to investigate, but Oliver brushes him off, telling Roy to stay away, as he is already on the case albeit from a different angle. Frustrated, Roy says he and his friends will investigate themselves, so Oliver shoots Roy in the leg to prevent him (and possibly Thea) from getting further involved.
Oliver tracks the thief to an A.R.G.U.S bunker,where the robber reveals that he is only a follower of "his brother" who "saved" him with the serum. Oliver fights him, losing, and is badly injured and injected with an unknown substance. In order to save Oliver's life while not revealing his identity, Diggle and Felicity abduct Barry and ask him to help find an antidote.
Meanwhile, Moira returns to the public eye and is not welcomed in society or by Isabel Rochev at Queen Consolidated. Not only that, but Malcolm has begun to put pressure on Moira to tell Thea that he is her father. Moira tells Malcolm she refuses to be afraid of him anymore. Instead, she has contacted Ra's al Ghul, who wants to kill Malcolm for breaking the League of Assassins' code of honor by destroying the Glades. As a result, Malcolm leaves and warns her that the issue regarding Thea is not over.
In a flashback to the island, Oliver, Sara, Slade, and Shado travel the island using the amulet to find the sub in a fissure on the island, and find the serum. They have the serum but no sedative, but since Slade will die without it they inject him anyway. Just before they do, Slade apologizes to Shado for not revealing he had feelings for her. When they inject Slade with it his heart stops, just as Dr. Ivo arrives at the sub with his men.
- This episode introduces the character of Barry Allen, who in the comics is the civilian identity of the Flash. This marks the first time a major hero from the comics is shown in the series.
- During the first scene involving Barry Allen, he can be seen holding a magazine questioning the S.T.A.R. labs particle accelerator's safety. Barry and Felicity later see a news broadcast on the same subject.
- This episode is notable as being the first episode of Arrow to hint at super-human abilities; previously the show was considered to be extremely grounded.
- In one shot, the name "Kord" was seen on lab equipment in a Queen Consolidated lab. This is likely a reference to Kord Industries and Ted Kord.
- Barry mentions seeing a blur during his mother's murder; this references the archenemy of The Flash, known as Professor Zoom or Reverse-Flash, who in the comics kills Barry's mother the exact same way.
- Lightning and thunder is heard while Barry is by the shelf holding chemicals. This is a reference to how Barry Allen, in the comics, got struck by lighting and was doused in chemicals therefore getting his powers.
- This episode marks the 7th main or recurring character to discover Oliver is The Arrow. The others being Diggle, Helena, Felicity, Tommy, Malcolm, and Sara.
- The Count's psychiatrist and orderly at the mental institution, and later the Count himself, also discovered the truth, but didn't live very long after making the discovery.
Preparation ran from September 19 until September 27, 2013. Shooting ran from September 30 until October 9, 2013.
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