The Once and Future Flash

"The Once and Future Flash" is the nineteenth episode of the third season of The Flash, and the sixty-fifth episode overall. It aired on April 25, 2017.

Synopsis
Barry travels to the future to find out Savitar's still unknown identity in the hopes of saving Iris. Upon his arrival in 2024, Barry encounters future versions of his Team Flash friends, who, following the death of Iris, have become very different people, profoundly impacted by their showdown with Savitar. It will be up to Barry to infuse a sense of hope back into the team, as he proves to be the hero his future friends have been waiting for. Meanwhile, back in 2017, the hunt for Killer Frost continues.

Trivia

 * This episode aired on April 25 and takes place in 2024, the date on which the Flash is said to vanish in a crisis, as published in a newspaper in the Time Vault.
 * However, the date Barry arrives in the future is February 3, 2024.
 * The title is a nod to an infamous inscription purported to be on the tomb of the legendary King Arthur in Le Morte d'Arthur by Sir Thomas Malory.
 * While hiding from Killer Frost, H.R. and Cisco sing "Swing Low, Sweet Chariot", a negro spiritual popularized by Eric Clapton.
 * In the future of this timeline, Barry and his friends' fate is revealed.
 * It is revealed that in the battle that Cisco vibed between him and Killer Frost earlier in the season, she managed to freeze Cisco's hands and shattered them, effectively ending his powers. Later, he replaces his hands with mechanical parts.
 * Killer Frost is put in the metahuman prison and an older Julian is in charge to keep her alive.
 * Wally is revealed to have gone after Savitar by himself after Iris' death. Unfortunately, he lost the fight and Joe found him the next day with a shattered spine.
 * Joe still does not get over with Iris' death and regularly visits her grave.
 * HR Wells is a celebrated author with a book titled "The Streak vs Mr. Reflecto" and reinvents CC Jitters into HR Jitters.
 * Barry stops being The Flash sometime later after he defeated Savitar. He shut down the S.T.A.R. Labs and abandons his friends and left them without support.
 * This episode actually shares many similarities with the DC's Legends of Tomorrow episode . Both episodes take place in the future and feature an older broken hero (Barry Allen in The Flash and Oliver Queen in DC's Legends of Tomorrow) living as a recluse in the remnants of their old hideout while a group of villains control their city and return to action when their former teammates inspire them and make them realize that their respective city will always need them, ending with them reforming their team and cleaning up their old hideout.