The episode opens during a lightning storm, with something slamming on a metal door or wall in a warehouse. Two security guards prepare for the worst but eventually waste a whole lot of bullets before being set upon by a man moving too fast and hitting too hard for them to stop--or survive.
At Queen Consolidated, Felicity welcomes Moira back awkwardly, and then Moira and Ollie head into a Board of Directors meeting. Isabel is not happy with that, and calls Oliver into the other room, saying that Moira presents a huge image problem for the company. She leaves, and Diggle and Felicity come to inform him about a break-in.
Outside in the rain, someone exits a train, shielding his head from the rain with a science magazine featuring the S.T.A.R. Labs particle accelerator. He looks at his watch and curses.
Inside, Officer Lance is explaining the break-in to Oliver, saying that the equipment taken was heavy and the doors were heavy duty: someone would have needed multiple people and probably some machinery to commit this crime, so they're at a loss as to why the security camera picked up only one masked man. Barry Allen enters, telling them that he believes it was just one man with enhanced strength and speed.
Allen introduces himself as being with the Central City PD's CSI unit and says that he's working on a case with similar elements. He explains why he believes it was someone with enhanced strength. He also suggests that what was stolen was likely an industrial certrifuge. He explains why he believes all of this based on some surface level evidence. Lance tells Felicity to fill Arrow in on what's happening.
On the island, they're still working toward the sub. Slade is badly hurt, and Oliver has to explain to Shado who Sara is. Shado is repulsed by the fact that he cheated on Laurel with her sister, and moves silently away from Ollie to help Slade walk.
Back at Queen, Diggle confirmed Barry's theory about the centrifuge and Felicity finds security camera footage from across the street that proves he was right about the super-strength, too. Barry shows up, and Felicity is going to set up his mobile lab at Queen so that he and Felicity can share information. She tells Oliver she thinks they need his help. Oliver (very) reluctantly agrees. Diggle immediately tells him that he believes Ollie knows more than he's letting on, and Oliver tells him to pray he's wrong.
At home, Oliver tells Moira that they need to put the trial behind them by throwing a welcome back party for Moira.
At the crime scene, Barry and Felicity are looking for forensic evidence that will lead them to the intruder. As they're waiting for the computers to process something, Barry asks Felicity about her experiences with the vigilante. He speculates that green only makes sense if he trained in the forest or jungle, where that's a better camoflage than black is in the city. He also suggests a new makeup of the arrowheads that the vigilante could use to improve his penetration, and telsl her he thinks Arrow has partners--at least one hacker. She asks why he's so interested and he admits that he wonders whether the vigilante could have caught the man who killed his mother when he was eleven.
Sin calls Roy to come see her in the alley behind Verdant, asking for help finding a friend who has gone missing. Thea tells Sin that Roy will do it and Thea will come with.
Merlyn comes to the mansion, menacing Moira, telling her that she has to tell Thea about her parentage soon. He says that after their affair, he went to find himself, which he did in Nanda Parbat; he tells her that in a way, he's only the Dark Archer because of her. She begs him not to make her tell Thea, but he tells her she has to or else.
Back at the crime scene, Barry and Felicity tell Oliver that they've tracked down a delivery truck stolen from a sugar factory based on waste sugar found in the dirt from the intruder's boots. The same truck was just used to rob a blood bank. Oliver mentions the similar case that Barry is working on and Barry stammers a bit.
Oliver tells the he's going to inform the police about the truck, then he leaves with Diggle, telling Diggle that Barry is hiding something and to find out what. Diggle asks when he's going to tell him what exactly they're up against.
On the road, Oliver's motorcycle catches up with the sugar truck, being driven by a masked man--it's Cyrus Gold, as seen last episode. Oliver jumps onto the moving truck and keeps hitting Gold, but can't seem to make much of an impact. Gold shatters the truck's windshield to pull Oliver in, then punches him out onto the road. Oliver, having stuck Gold with an arrow, takes the arrow with him.
Back in the Arrowcave, Oliver tells Felicity and Diggle that he's seen people like that before--on the island. He admits the Japanese Miracle serum was used there but tells them that everyone involved is now dead. He hands Felicity the bloody arrow he took out of Gold, telling her that he needs her to run a screen on the blood and see if they can't figure out more about the operation to synthesize the drug. She tells him that she and Barry will get right on it. Diggle tells Oliver after she leaves that Allen isn't who he says he is.
On the island, Slade is fading fast. They find a pond in which the submarine is partially submerged.
Felicity tells Barry that they have to figure out what kind of sedative was given to Gold to keep his body from rejecting the serum, which requires admitting that she has the arrow. She dances around how she got it, but he's just impressed they're all working on the same case. Felicity asks him to be her guest to a work function.
At Sin's friend Max's apartment, Roy and Thea figures out that he went missing the day of a blood drive. They leave the apartment and are spotted by one of Brother Blood's people, who calls it in, and Blood tells them that if they're looking for Max, they should find him.
Back at the office, Barry Allen and Felicity are debating whether S.T.A.R.'s particle accelerator is a good idea or not when Oliver storms in to call Barry a liar; he's an assistant, not a full CSI tech, whose bosses don't know he's there because there is no similar case. Barry admits that the "similar case" is his mother's years-old cold case. Oliver tells him that the police solved it and arrested his father, and Barry says that his father didn't do it--that something came into the house one day and that a tornado with a face took him instantly blocks away at the time of the attack. He says that nobody ever believed him, thinking he was covering for his father, but that it was real and that's why he's fascinated by cases involving strange phenomena. He apologizes for lying and leaves, but has drummed up enough sympathy that Felicity is upset with Oliver for making him come clean.
That night at Moira's party, attendance is sparse. Felicity accuses Oliver of having overreacted earlier but tells Oliver that he should apologize to Moira, not her, for having engineered a spectacularly failed party. She goes into the room anyway to be kind to however few guests there are. Thea is talking to Roy when Sin calls, telling him that they found Max's body and that she neeeds him. He leaves, lying to Thea about his reasons. Oliver surprises Felicity by bringing Barry to the party. The two dance and chat awkwardly. At the bar, Oliver and Isabel share a drink. She tells him that she tried to warn him but that she really is sorry. He leaves and Moira approaches Isabel, intimating she knows they were intimate. Oliver breaks up their chat.
Back on the island, Slade finally slumps in a corner of the sub and is nearly dead; they decide to dose him with the miracle, even though they have no sedative, because that'st heir only chance at saving him.
At her party, a strange man is talking to Moira, telling her that "He says to tell you your information is gratefully accepted." Ollie and Thea come in to talk to her and Oliver has to excuse himself when Felicity calls. Moira tells Thea that she needs to tell her something, but she says only that she loves her.
Back at Queen, Barry is getting chewed out by his boss on the other end of the phone and promises that he'll be back in Central City tonight so that he doesn't lose his job.
Back at the scene where Max's body was found, the cops are telling Sin it was an overdose; she knows that can't be right because he donated blood the day he died. Roy sneaks in to take a photo of the body before the cops chase him off. The cop is the same guy who was staking out the house earlier.
Back at the Arrowcave, Felicity has found an A.R.G.U.S. facility--a disaster bunker where the bad guys are probably headquartering their operations. She tells Oliver that the way the arrowhead is bent, the perp's muscle density is too hard to be stopped with the arrows. Ollie says he'll do it, as he's done it once before, and she asks what if he can't.
Back on the island, Shado holds Slade's hand as he professes his feelings for her before he's injected and seemingly dies. Immediately afterwards, Oliver and company are taken by Ivo and his men.
Roy calls Arrow in to talk about Max, whose corpse was bleeding from the eyes. He realizes it's Miracle and tells Roy to stay away from it. Roy says that he has friends who can help him, that Arrow can't stop him, so Ollie shoots him in the leg with an arrow.
In her room, Merlyn approaches Moira; he tells her that she has no choice but to give him Thea. She says she's through being afraid of him. He asks if she thinks she can stop him--she says no, but Ra's al Ghul can. She researched Nanda Parbat and the League of Assassins, and sold him out to Ra's--his undertaking betrayed the League. Ra's, she says, want to kill Malcolm himself. She tells him he'd better run. He leaves.
At the A.R.G.U.S. facility, Gold starts hurling heavy objects and throwing haymakers at Arrow, who briefly slow him down by shooting arrows through his feet and into the floor. Ultimately though Gold gets the upper hand and hurls him into a rack where he's stuck with a handful of falling syringes.
Diggle and Felicity arrive to find him there, still alive but unconscious. They can't identify the syringes to find out what's poisoning him, so Diggle is going to call 911. Worried that doing so will compromise Oliver's identity, Felicity admits they can't save him--but says she knows who can.
At the train station, Barry has just missed the last train out for the night and has to sit and wait on a bench. Out of the blue, he's struck in the neck by a dart and wakes up in the Arrowcave. He's shocked to see that Arrow--whose face he can see, and recognizes as Oliver--is on a table in front of him, unconscious. Felicity comes around to him, asking him to save her friend.