- "Why the charade, running around dressed like the Flash?"
"To give people hope, Detective."
"So I could rip it away from them."
- —Joe West and Zoom
Equipped with the tachyon device, Barry believes he is fast enough to stop Zoom and wants to open the breaches to catch him. Harry strongly advises against that plan but after Barry convinces him, Harry reveals that Cisco has the power to re-open the breach. Meanwhile, Hunter Zolomon's painful story on Earth-2 and his origin as Zoom are revealed.
- Grant Gustin as Barry Allen/The Flash
- Candice Patton as Iris West
- Danielle Panabaker as Dr. Caitlin Snow
- Carlos Valdes as Cisco Ramon/Vibe
- Keiynan Lonsdale as Wally West
- Tom Cavanagh as Dr. Harry Wells
- Jesse L. Martin as Detective Joe West
- Tatayana Forrest as Ashley Zolomon
- Shaine Jones as James Zolomon
- Octavian Kaul as Young Hunter Zolomon
- Brian McBride as CPS Man
- Valeria McNicol as Caretaker
- Ryan Handley as Zoom
- Lucas Wolf as Man in the Iron Mask
- Tony Todd as Zoom (voice)
- Amina Elkatib as Young Iris West (archive footage)
- Michelle Harrison as Nora Allen (archive footage)
- David Hayter as King Shark (archive footage)
- Allison Paige as Eliza Harmon (archive footage)
- John Wesley Shipp as Henry Allen (archive footage)
- Logan Williams as Young Barry Allen (archive footage)
In flashbacks, events from 15 years ago are shown, when Reverse-Flash killed Nora Allen, and from Earth-2, where Hunter Zolomon's father, James Zolomon, argues fiercely with Hunter's mother Ashley Zolomon, which ends with James shooting Ashley in front of him. Hunter is taken to an orphanage.
In the present day Earth-1, Barry, Cisco, Caitlin and Iris are preparing to run a test on the tachyon device to see if Barry's speed can increase with it. The test proves to be success, with Barry running with extreme speed without consequences. He even travels through a dimensional breach and reappears less than a second later (during this brief time he ended up on Earth-38 and became friends with Kara Danvers/Supergirl). When he returns to S.T.A.R. Labs, no one even realized he traveled to another world entirely. Satisfied that he is now faster than Zoom, Barry intends to reopen a breach to Earth-2. Harrison Wells objects, and claims the breaches can't be reopened but he agrees to make the tachyon device smaller. Iris goes on a date.
At Joe's place, Wally arrives with laundry and informs he has no place to live on campus. Barry gets the idea about opening the breaches with Cisco's powers, since his doppelganger, Reverb, was able to manipulate dimensional energy. They try it in the breach in the labs, but nothing happens. Wells goes to see Joe and attempts to get him to stop Barry from opening a breach, but Joe tells him, that once Barry sets his mind on something, there's no way to talk him out of it. Instead he tells Wells to help him.
Barry and Cisco find the strongest breach source in an abandoned mental hospital. Cisco is still unable to open the breach, until Wells arrives with his goggles. With the goggles, Cisco is able to open the breach, and Zoom also sees it and urges Cisco to keep coming. But Cisco stops and runs off.
Joe and Iris come to the labs to give help, and Barry tells Joe that Wally wants Joe to invite him to live with him. Caitlin and Iris talk about Iris' situation, as Caitlin has noticed she has been watching Barry with love for some time. Iris admits she didn't go on her date yesterday, as she has started to think that maybe she is truly meant to be with Barry, reasoning that they are married on Earth-2 and supposedly married in the future. Barry goes to see Cisco, and Cisco tells him he's not afraid of Zoom, but himself. He fears that if he keeps using his powers, he will end up becoming like Reverb. But Barry says that unlike Reverb, Cisco has friends here, who consider him family, and they will always stand by him. Cisco agrees to try again. Joe offers Wally a chance to live with him and he accepts.
Wells has succeeded in remaking the tachyon device, and the team begins to wonder, if Zoom's real name really is Jay. Caitlin tells the team, that there is no Jay Garrick on Earth-1, rather his doppelganger's name was Hunter Zolomon. The name alerts Wells, who then reveal, that Hunter Zolomon on Earth-2 was a convicted serial killer, who was given to an orphanage, when his father killed his mother in front of his eyes. He murdered 26 people, until he was finally caught and sent to a mental hospital. While being treated, the particle acceleration explosion affected him, giving him the Speed Force. Thus, Zoom was born. Barry gets an idea on how to defeat him.
Barry and Cisco return to the mental hospital, while Wells, Joe, Iris and Caitlin take shelter inside the Time Vault, as Zoom supposedly can't enter it. Cisco opens the breach and Zoom comes out. Barry runs and Zoom gives chase. Barry is able to run faster than Zoom and lure him into S.T.A.R. Labs. Inside, Zoom finds that Barry has set the room with dummies of his parents. They distract him long enough to allow Barry to trap him with meta-human chains. Zoom questions. how did Barry figure out his real identity, and Barry says he made a mistake, when he told Caitlin about his doppelganger. When it seems that Flash is victorious, Zoom manages to break free and escapes.
Barry, Iris and Joe return home, to find the place is ransacked. They find that Wally is missing, and that Zoom has him. He left a message: "Your speed for Wally's life." Zoom has Wally taken to his lair on Earth-2. Eventually, Barry finds no other choice, but to accept.
Cisco vibes, and calls to Zoom, who appears with Wally. The exchange is made, and Wells inserts the Speed Force Draining device into Barry's suit. The team questions Zoom, why did he do the things he did and how he was able to convince them, that there was two of him. Zoom reveals that he traveled to the past to meet himself, making Cisco realize he is a time remnant, like the Reverse-Flash. His past self agreed with his plan to steal Flash's speed in order to cure him, and also inspiring hope in people, so that he could then snatch it from them. Barry uses the treadmill so that his Speed Force can be extracted. All of his speed is taken and Zoom injects it to himself. He is thrilled and proceeds to kill Barry, until Caitlin talks him out of it. He then escapes, taking Caitlin with him.
- While Barry tests out the tachyon device, he travels through a breach and comes out through another breach, with the leitmotif blending in with the background music. Once Barry returns to S.T.A.R. Labs, he asks, "How long was I gone?", but neither Cisco nor Caitlin pay attention to the statement. This is a reference to "", an episode of in which crossed over. This episode took place within this very brief moment.
- Coincidentally, both take place at the eighteenth episode of their own series.
- Strangely, when Wells tells Barry that there is no way back to another Earth, Barry doesn't tell him about how he had accidentally run to another Earth, which had happened earlier that episode (most likely due to his uncertainty about how that happened and his inability to precisely create such a breach).
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