- "Just one thing that occurs to me, I don't need you, do I? Not this you certainly. Oops, you probably should have thought of that before you came back here. Shame... you ran all the way back here just to die."
- —Eobard Thawne claiming that he doesn't need the future Barry to travel back to his time period.
|After the shocking revelation that Zoom is Jay Garrick, Barry is more determined than ever to get back to Earth-2 to stop Zoom forever. Desperate to find a way to increase his speed, Barry decides to travel back in time and masquerade as his earlier self in order to get his nemesis, Dr. Harrison Wells/Eobard Thawne, to teach him how to run faster. However, things don't go as planned and Barry is stunned to face familiar foe Hartley Rathaway, old friend Eddie Thawne, as well as the real danger of time travel. Iris makes peace with her past in order to embrace her future.|
- 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. Harrison "Harry" Wells and Professor Eobard Thawne
- Jesse L. Martin as Detective Joe West
Special guest star
- Guy Christie as Cop
Barry continues to find ways to increase his speed, now that Zoom's identity has been revealed. The entire team devote their time to help Barry gain speed, but they continue hitting dead ends. Barry returns home to have dinner with Joe and Iris, and the yet-to-be-here Wally. Wally announces that his project was so good, his professor was willing to work with him again. He wants to improve in his studying, and jokes that he wishes he could ask Ferrari or Porsche for help, but they're dead. That gives Barry an idea to go back in time and consult Eobard Thawne for a way to increase his speed. Barry leaves the West house and goes to S.T.A.R. Labs to discuss his idea with the rest of the team. Wells initially forbids it, but then he realizes Barry has no choice. Barry is given a sedative for him to administer to his past-self.
As Barry moves to the past, he spots a Time Wraith flying towards him. This distraction causes him to end up traveling back a little too early to his past-self and Hartley Rathaway's first altercation in front of Rathaway Industries, rather than the battle at the dam. The past Barry notices future Barry and after a brief struggle, future Barry manages to sedate past Barry and switches the emblems on their suits.
Barry then returns to S.T.A.R. Labs to meet with Harrison Wells/Eobard Thawne, and brings his conundrum with the speed equation to him. After some convincing, Thawne agrees to help him with the equation.
In the pipeline, Cisco and Caitlin are locking Hartley into a cell. Knowing how Hartley escapes, Barry asks Cisco to scan for explosives. The three find his hearing aids to be explosive, and they make Hartley remove them. To counteract his tinnitus, Eobard suggests they play music, before rolling away.
At CCPD, the Time Wraith attacks, tearing apart Barry's lab before appearing at the top of the staircase in front of the entire department. The force's shots do nothing to harm the wraith. It then leaves through the window to find Barry. Barry cleans up his lab as Eddie enters to ask Barry about the wraith, leaving him speechless upon seeing Eddie again for the first time in a year. Joe misinterprets this as awkwardness between him and Eddie due to his love for Iris but Barry states that he's moved past it.
Back at S.T.A.R. Labs, Eobard calls Barry back into his lab for information on his speed equation and promptly knocks Barry unconscious while his back is turned. Barry awakens in Eobard's secret room handcuffed to his wheelchair. Eobard knows that it's Barry from another timeline as the Time Wraiths only chase speedsters when they travel through time. His suspicion is further confirmed when he notices Barry doesn't flinch upon seeing him walk and use his own speed, as well as when Barry attempts to phase through the handcuffs, since Eobard had not yet taught past Barry how to phase through objects yet. He becomes upset when he believes that his plan fails if there is a future Barry still alive, but Barry convinces him that it is because of the singularity opened by Eobard's returning home that has sent him back, as he needs to learn how to stop it. He remarks that he doesn't need the future Barry to travel back to his time and prepares to kill him with his vibrating hand until Barry forces him to help with his speed by saying that he left a letter on his past self that reveals how to beat Eobard, meaning he will never make it back home.
In the pipeline, Cisco is playing Rick Astley's "Never Gonna Give You Up" on full blast to torment Hartley into revealing how the gauntlets work. The Time Wrath shows up at S.T.A.R. Labs in pursuit of Barry and forces Cisco and Caitlin to hide in the cell with Hartley for safety. After convincing Cisco to trust him, Hartley is able to rig the gauntlets to emit a destructive signal in the wraith's frequency, hurting its ears as it flees.
After Barry and Eobard reconvened with Caitlin and Cisco, they remarked that they would have died without Hartley and deduced that the Time Wraith is after Barry. Cisco and Caitlin worked on repairing Hartley's gauntlets while Eobard and Barry gathered information on the Time Wraith.
At CCPD, Barry feigned making a birthday video for Iris to convince Eddie to record a video professing his love for her. While Cisco and Caitlin were commenting that Hartley was the only one able to make the necessary repairs to the gauntlets, past Barry showed up at S.T.A.R. Labs. Cisco and Caitlin were confused, but future Barry revealed to them that he time-traveled. Eobard then took future Barry aside to give him the answer to Barry's efforts to increase his speed.
Before future Barry went back to his own time, he told Cisco that Hartley knows about Ronnie and switched emblems with his past self. He then started running in the particle accelerator but the Time Wraith chased him, slowing him down. Past Barry then ran after the Time Wraith and kicked it off course allowing future Barry to run through the wormhole.
Once Barry returned to the present, Cisco and Caitlin used a sonic gun on the Time Wraith but it failed. The wraith grabbed onto Barry sucking the life out of him, but Hartley showed up and used his gloves to destroy the wraith. Hartley then left for dinner with his parents and Barry was surprised at the change in timelines. They went to the console in the Time Vault and accessed Eobard's information on speed enhancement. Barry then went to console Harry about his strained relationship with Jesse. Later, Barry gave Iris the video he recorded with Eddie and Iris sheds a tear.
- Barry traveled back in time to the events of "".
- Hartley Rathaway was last seen in "".
- To keep Hartley's ears from ringing with pain while he makes new hearing aids to replace the explosive ones, Cisco plays Rick Astley's "Never Gonna Give You Up" on a loop; it also doubles as torment.
- Eddie Thawne was last seen in "", albeit in a dream sequence.
- Eobard Thawne impersonating Harrison Wells was last seen in "" and in a video sequence in "".
- Past Barry is vulnerable to tranquilizers when three episodes prior, in "", he could neutralize their effects. Although this Barry was caught off guard and it is likely S.T.A.R. Labs has a better idea of what it takes to knock out a speedster.
- The idea of a "speed equation" is used in the comics by superhero Johnny Quick, the father of Jesse Quick in the comics. What Barry writes on the board, "3x2(9yz)4a", is the formula comic book Jesse Quick uses to tap into the Speed Force.
- In "", Hartley said "Your silence speaks volumes" to Eobard in Latin. In this episode, Eobard is the one to say it to Hartley, as a result of Barry changing the timeline.
- Eobard knows the future Barry is not his Barry, as he hasn't taught him how to phase through objects yet, which he does in "", six episodes after "".
- The Time Wraith is visually similar to Dementors from the Harry Potter franchise, with Cisco even referring to the Time Wraith as "Dementor", and Hartley exclaims "Holy Harry Potter!" upon seeing it for the first time.
- During the time period of "", which aired in late January, Eddie says Iris's birthday is in two months, which would place it sometime in March or April. However, Iris's birthdate is later shown in "" to be June 24.
- Barry takes a video from Eddie (from the past) to Iris (in the present). In the present timeline, she shed a tear while watching it. This is a reference to the 2011 Flashpoint storyline, where Thomas Wayne from an alternate timeline sent a letter (with the help of Barry Allen) for Bruce Wayne from the new timeline, who shed a tear while reading it.
- Barry made a major change in the timeline as Hartley Rathaway is on the side of Team Flash and is also on good terms with his parents, who had previously disowned him because he came out as being gay.
- Toward the end when Barry says his trip to the past was interesting, Caitlin echoes Cisco's line from the beginning of the episode, "Good interesting or bad interesting? 'Cause that (word) could go either way."
- Cisco Ramon refers to the Time Wraith as "Inky", one of the four ghosts featured as enemies in the arcade game Pac-Man.
- Barry reveals that time travel exists, which the team didn't originally learn about until "", three episodes later.
- Before the Time Wraith enters S.T.A.R. Labs, when Caitlin is using the computer, she notices a new phone model which belonged to the Barry from the future, which made her astonished.
- The title is a pun in that it is technically a flashback, as it involves past events, and also involves the Flash (Barry) literally going back to the past events, via time travel.
- In the scene where Iris is watching a video made by Barry during his time in the past, her finger position when holding the phone changes between shots.
- The Time Wraith attacking the Pipeline cell (and therefore Hartley, Cisco and Caitlin) makes no sense as they are not speedsters.
- Thawne claims he was on the lookout for Time Wraiths when he time traveled, but they only notice speedsters using the regular speed force; Thawne uses his own Negative Speed Force.
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