Flash Back

"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.

"Flash Back" is the seventeenth episode of the second season of The Flash, and the fortieth episode overall. It aired on March 29, 2016.

Synopsis
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 arch-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.

Plot
Barry continues to find ways to increase his speed, now that Jay'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 Wests' house and goes to STAR 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 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 (past Barry didn't know he had super speed), and when Barry attempts to phase through the handcuffs, since Eobard had not yet taught past Barry how to phase through objects yet. He remarks that he doesn't need the future Barry to travel back to his time and prepares to kill him until Barry forces him to help with his speed by threatening to give past Barry a letter on 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.

Trivia

 * The Flash crossed over with Supergirl in the Supergirl episode, . This crossover takes place chronologically between The Flash ' s "Flash Back" and.
 * The Supergirl episode "Worlds Finest" actually spoils part of this episode because in "Worlds Finest", Barry has a tachyon emitter on his chest to get faster. In this episode Barry gets the information needed to create a tachyon emitter. Despite the fact that this episode was aired after the Supergirl crossover, it takes place chronologically before "Worlds Finest".
 * Hartley Rathaway was last seen in.
 * Eddie Thawne was last seen in, albeit in a dream sequence.
 * Eobard Thawne impersonating Harrison Wells was last seen in Fast Enough and in a video sequence in.
 * Eobard Thawne 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 . On the other hand, past Barry is vulnerable to tranquillizers when three episode priors, in , he could neutralize their effects, although it is likely STAR Labs has a better idea of what it takes to knock out a speedster.
 * The Time Wraith appears to be visually based on the Black Flash in the comics.
 * The Time Wraith is compared to Dementors in the Harry Potter universe. Cisco even refers to Time Wraith as "Dementor", and Hartley exclaims "Holy Harry Potter!" upon seeing it for the first time.
 * 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 that formula.
 * Barry takes a video from the Eddie (from past timeline) to Iris (from present timeline). In the present timeline, she shed a tear while watching it. It's a reference to the 2011 Flashpoint comics, where Thomas Wayne from an alternate timeline send a letter (with the help of Barry Allen) for Bruce Wayne from current timeline. He 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 he seems on good terms with his parents because in the previous timeline Hartley's father disowned him because he came out as being gay. It seems this isn't the case anymore.
 * Cisco Ramon refers to the Time Wraith as "Inky", one of the four ghosts featured as enemies in the arcade game Pac-Man.