- "You will have a father - your real father. Wells has messed with our lives long enough. This is why you became The Flash, Barry. To put things right. You saved a lot of people's lives this past year. Now it's time to save yours."
- —Joe West encouraging Barry Allen to save his mother
|Wells presents Barry with a life-changing choice. Dr. Martin Stein and Ronnie Raymond return to help the S.T.A.R. Labs team with this final fight.|
- Grant Gustin as Barry Allen/The Flash
- Candice Patton as Iris West
- Danielle Panabaker as Dr. Caitlin Snow and Caitlin Snow/Killer Frost
- Rick Cosnett as Detective Eddie Thawne
- Carlos Valdes as Cisco Ramon
- Tom Cavanagh as Professor Eobard Thawne/Reverse Flash
- Jesse L. Martin as Detective Joe West
Special guest stars
- Matt Letscher as Professor Eobard Thawne/Reverse-Flash
- Robbie Amell as Ronnie Raymond
- Michelle Harrison as Nora Allen
- Patrick Sabongui as Captain David Singh
- John Wesley Shipp as Dr. Henry Allen
Special appearance by
A bitter Barry questioned Eobard on his origins and motivations. Eobard told Barry he hated the future Barry for reasons undisclosed even though he had grown to care about the present Barry to a degree.
In the year 2024, he learned The Flash's true identity as Barry Allen and traveled back in time to kill the younger Barry. However, he was followed by the Future Barry and they fought in his house. In order to protect him, the Future Barry grabbed his younger self and retreated twenty blocks away. An enraged Eobard then killed Nora in hopes the trauma would prevent the younger Barry from pursuing his destiny and becoming the Flash. However, since Eobard lost his powers shortly after and was stranded centuries before his own time, he was forced to create, ironically, the very enemy he sought to destroy. Eobard offered to let Barry prevent Nora's death if he created a wormhole for both him and Barry to return home to the future. Meanwhile, Eddie is torn about learning about Iris' fate to marry Barry. However, Professor Stein told Eddie that he, being Eobard's ancestor worked in the same city as his descendant, ultimately made him an anomaly to Eobard's plan known as coincidence. Because of this, despite Eobard's self-centered personality, he had to keep Eddie alive above all else until he had a child to carry on the Thawne name, which centuries later would lead to Eobard being born. Inspired by Stein's words, Eddie decides to forget about the supposed future and date Iris once again. Iris happily agrees. Still torn over whether to save his mother and risk everything he already has, Barry visits his father in prison but gets told that he shouldn't go back to save Nora due to the changes it would make to the future. Furthering Barry's doubt,
Once the preparations were set for Barry to travel back in time, he traveled back to the night Nora died. However, despite this, Barry's alternative future self saw Barry and signaled him to not to interfere and proceeded to transport his younger self 20 blocks away. Barry, in tears, obeyed. He spoke to Nora once last time, revealing his identity to her, and assured her that he and Henry are okay in the future.
In the present, Eobard gets into the time machine Cisco made, remarking that Rip Hunter first designed it. Cisco warns him never to return. Suddenly, a helmet with wings on top comes out of the wormhole, which Eobard takes as his signal to leave. However, Barry returns, destroying the time machine with a supersonic punch. This enrages Eobard, who realizes that Barry didn't save Nora and demands to know why not, when he could've had the life he wanted. Barry retorts that he already has the life, friends, and family he wants. Barry's unannounced return causes Professor Stein to order Caitlin and Ronnie to manually shut down the generators. Caitlin succeeds in shutting down the generators and the wormhole is closed just before the 1:52 runs out.
With his Time Sphere destroyed, unstable speed and motives revealed, Eobard has no way to return 'home'. Enraged beyond belief, he charges at Barry and they proceed to fight, which ends in Eobard beating Barry, pinning him to the wall and announcing "Just so we're clear, after I kill you, I'm going to kill them (Team Flash) and then I'm going to kill your father! I always win, Flash!". However, Eddie shoots himself in the heart. As Eddie Thawne is a direct ancestor of Eobard Thawne, his death causes Eobard to be erased from existence. This creates a paradox: since Eddie did not leave behind any children to continue the Thawne name, Eobard is never born and therefore the entire timeline that he created by killing Nora Allen could have never been created.
As he is erased from existence, Eobard reminds Barry that he might not be able to get along without him after years of controlling his life. As Iris and Joe mourn Eddie, the paradox causes the wormhole to reopen and start expanding, pulling Eddie's body into it. The singularity rises up out of S.T.A.R. Labs, feeding and growing in size and strength. Despite a warning from Stein that "it cannot be stopped!" and will continue to consume even after the Earth is gone, Barry runs up to the it in a bid to neutralize it by running around the debris floating into it in the opposite direction with equal rotation, just like he unraveled the tornado in Episode 1.
Preparation began March 19, 2015 and ran until March 27, 2015. Filming began March 30, 2015 and ran until April 10, 2015.
- The DVD release of season one includes several deleted scenes for this episode. Among them is a scene where Eobard reveals to Joe that the device he used to steal Harrison Wells' identity in "" also stole Wells' memories and feelings.
- One of Barry's Speed Force visions shows a giant robotic foot smashing through a roof while Rip Hunter's team fight Savage's army; this is test footage for that was shown in a trailer.
- Eddie Thawne and Eobard Thawne both seemingly die in this episode, only for Eobard to return for several episodes the following season, as the main antagonist of Season Two of , main antagonist in the Crisis on Earth-X crossover, and a hidden antagonist in Season 5. This mirrors the events of the season 1 finale, "", where Tommy Merlyn and Malcolm Merlyn both seemingly die, only for Malcolm to return for a few episodes the following season and reappear later in the series.
- This episode marks the final appearance of Rick Cosnett as a series regular.
- This marks the first appearance of the Anti-Monitor Crisis.
- Despite being a cameo, this marks the first time in an episode of where Captain Cold appears without the word "Rogue" in the title.
- This is also the first time Captain Cold has appeared in two consecutive episodes as previously, there would be six-episode gaps before his reappearance.
- Eobard says "run, Barry, run", a callback to when he said the same line in "", when Barry was fighting Clyde Mardon.
- Among the things that Martin Stein suggests that he will shout upon time travel, he says "Excelsior", the catchphrase of famous Marvel comic book writer, Stan Lee.
- Eobard reveals to Cisco that he can see through vibrations in the universe, and that a "great and honorable destiny awaits" him. This is a reference to his comic book alter ego, Vibe, which also ends up becoming his alter ego on the show.
- Eobard directs Cisco to coat the Tungsten panels for the Time Sphere with "cobalt resin". This is a reference to Cobalt Blue, aka Malcolm Thawne from the comics, who is the present-day ancestor of Eobard Thawne, making him Eddie's counterpart from the comics.
- Cisco sarcastically uses the phrase "so long, and thanks for all the fish", the final message dolphins give to humans before leaving Earth in The Hitchhiker's Guide to the Galaxy.
- Barry only being able to stay in the past for 1 minute and 52 seconds is a reference to The New 52.
- Cisco wishes Barry luck by saying "may the Speed Force be with you", referencing the similar line from the Star Wars saga.
- In his Speed Force visions, Barry sees several references to the comic books:
- The Flash Museum is from the comics, in which it houses exhibits of Barry's greatest achievements, including suits and memorabilia of his allies and villains.
- Barry sees himself as a prisoner in Iron Heights Prison, handcuffed and wearing a jumpsuit, speaking with an unknown person. This is a reference to The Flash story arc The Trial Of The Flash!, in which The Flash is charged with the murder of the Reverse-Flash.
- Barry glimpses the future of Caitlin Snow, who is reminiscent of Killer Frost and shoots ice out of her hands.
- Eobard mentions that Rip Hunter created the time sphere he is using to return to his home time. Hunter is a former Time Master from 2166, who later stars in the Flash spinoff .
- A winged helmet was tossed out of the wormhole as Eobard Thawne was attempting to go home to the future; this is the helmet worn by the Golden Age Flash of the comics. In "" it was revealed that Hunter Zolomon is the Flash of Earth-2 under the guise of Jay Garrick, and that this helmet belongs to him; he claimed to have lost it when the singularity opened during his final "fight" with Zoom.
- Professor Stein says the event has an energy level of 6.7 Tera-electronvolts (TeV), and thus can't be stopped. In reality, an electron volt is a tiny amount of energy, equal to 1.073 x 10−18 Joules. 6.7 x 1012 eV is still much less than 1 Joule, which is the amount of energy it takes to lift 1 kilogram 10 centimetres off the ground.
- It's possible the writers were equating electron volts with volts, which would be a more impressive amount of energy: 6.7 Teravolts would be around 67,000 times as much energy per unit of charge as is commonly found in a bolt of lightning.
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