Fast Enough

"You can go back and save your mother, you can prevent your father from going to prison, you can reunite the Allen family."

- Eobard Telling Barry about his opportunity

"Fast Enough" is the twenty-third episode of the first season of The Flash, and the twenty-third episode overall. It aired on May 19, 2015.

Synopsis
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.

Plot
Barry questions 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. For undisclosed reasons, the Future Barry grabbed his younger self and retreated twenty blocks away. Eobard, being so angry killed Nora in hopes the trauma would prevent him from being the Flash. However, since Eobard lost his powers, he was forced to create what he sought to erase from existence, The Flash. Eobard offered to let Barry prevent Nora's death in exchange if he created a wormhole for both him and Barry to return home to the future and travel to the past respectively.

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 Prof. Stein's words, Eddie decides to forget about the supposed future and date Iris once again. Iris happily agrees.

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 told him to not to interfere and proceeded to transport his younger self 20 years into the future. Barry, in tears, obeyed. He spoke to Nora once last time, revealing his identity to her, and returned to the present and prevent Eobard from returning to his time, deciding 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 ended in Eobard beating Barry, pining him to the wall and announcing "...once I kill you, I will kill all of [Team Flash] and then I'll kill your Father...I always win, Flash!" Eddie however, shot himself in the chest to erase Eobard from existence. As Eddie is a direct ancestor of Eobard Thawne/Harrison Wells/Reverse Flash, his death causes Eobard to be erased from history. 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. This paradox is what causes the second singularity to reappear: nature abhors a vacuum and will naturally eliminate anything unstable. The black hole rises up out of S.T.A.R.Labs, feeding and growing in size and strength. Despite a warning from Prof. Stein that "it cannot be stopped!", Barry ran up to the black hole in hopes of neutralizing it by running around it in the opposite direction with equal rotation, just like he unraveled the tornado in Episode 1.

Production
Preparation began March 19, 2015 and ran until March 27 2015. Filming began March 30, 2015 and ran until April 10, 2015.

Behind the scenes

 * Despite being a cameo, this marks the first time in an episode of The Flash where Captain Cold appears without the word "Rogue" in the title.
 * This is also the first time Captain Cold has appeared in a consecutive episode as previously, there would be six-episode gaps before his reappearance.
 * Eobard urges Barry to "run, Barry, run", a callback to when he said the same line in, when Barry was fighting Clyde Mardon.