A New World (The Flash)

Chester Runk gives Allegra Garcia her first suit and Iris receives a Pulitzer nomination. Barry Allen gets transported to the day Nora Allen dies. He goes to Joe West for help and later sees his parents, but is knocked unconscious by Eobard Thawne. While investigating, Joe finds a blue crystal and becomes possessed. After being hospitalized and having lunch with his parents, Barry is approached by Thawne, who gloats about him being unable to stop his plan to kill Barry's younger self. Barry returns to the hospital and says goodbye to his parents. The possessed Joe then attacks him, stating that he is not the new Negative Speed Force avatar but still wants to correct the imbalance Barry's legacy leaves on the timeline. Barry frees Joe and returns him to his car, but the crystal disappears. Barry then participates in the fight in which Thawne kills Nora Allen, warning his earlier self not to interfere. Afterwards, Barry tells Thawne that he has made peace with Nora's death before he is transported away again and Thawne loses his powers. Elsewhere, a man resembling Eddie Thawne is struck by lightning at Mercury Labs, finds a file about Eddie's death, and wonders who he is.

Chester finds traces of cobalt where Barry was taken. Mark returns, but he attacks members of team Flash and is later shown to be possessed by the crystal. After he gasses CCC Media, Khione rescues Taylor and Aariz by growing plant cells in their bodies, but their skin turns green. They flee in horror, and Mark calls Khione an abomination. The Speed Force defends Iris from Mark and explains that he became a vessel for the Negative Speed Force. After the Speed Force convinces her to embrace her power, Khione frees Mark by disintegrating and reassembling his body, but the crystal disappears. Barry returns as Iris goes into labor and Khione makes plans to leave Central City, but is pulled away again. Elsewhere, Malcolm Gilmore— Eddie's facsimile— goes to Captain Daisy Korber at the CCPD to learn about Eddie. After hearing a voice telling him to find Iris and seeing Eddie's memories, he breaks into CCC Media and digs up Eddie's grave, only to find it empty. He realizes that he is Eddie and pulls out the bullet he shot himself with.

Eddie, in a moment of rage, inadvertently sends Korber into a singularity. The Speed Force shows up to warn team Flash that Barry and the crystal are in 2049 before vanishing. At the Flash Museum, Eddie sees a girl resembling Nora West-Allen claiming to be his daughter, but the real Nora stops him. Team Flash 2049 finds that Eddie is subconsciously opening the singularities. Nora becomes possessed by the crystal and tries to tempt Eddie into accepting its power, but Barry appears and escapes with him. Eddie is angered by how his sacrifice didn't erase the Reverse-Flash and how he was forgotten while Barry lived and married Iris. In 2023, Cecile projects her consciousness into her 2049 self, but returns when she learns that she rarely visits Joe due to work. After Chester (at Khione's urging) consoles her, she returns and frees Nora from the crystal's influence. Eddie asks 2049 Iris to be with him again, but she rejects him. As the Negative Speed Force begins to fracture the timeline, 2049 Iris talks with Barry before he is pulled away again. Now in possession of the crystal, Eddie accepts its power and opens a portal.

Now a speedster known as Cobalt Blue, Eddie resurrects Eobard Thawne / Reverse-Flash, Hunter Zolomon / Zoom, Barry Allen / Savitar and August Heart / Godspeed and gathers them in the Negative Speed Force. Returning to 2023, they fight team Flash. Nora fights Savitar whom she stabs. Cecile adopts her future costume and codename Virtue, and defeats Godspeed. Allegra saves Chester from Eobard, and Jay Garrick steals Eddie's speed. Eddie retreats into the Negative Speed Force to gain more speed. Not wanting Eddie to die, Barry follows him in and convinces him not to become like Eobard and that they can create a better world by coexisting. Eddie destroys the crystal and says he is happy for Iris, and Barry returns to her at the hospital. Mark reveals that Chester is a metahuman with black hole powers. The next morning, Nora is born and Harrison Wells tells Khione to ascend as the natural order's protector. She bids farewell to Team Flash and returns her body to Caitlin (it's possible she has some of Frost and Khione's powers). At a celebration one week later, Barry apologizes to Caitlin for how they left things and Joe and Cecile get engaged. Barry unleashes a lightning bolt and chooses Avery Ho, Max Mercury and Jess Chambers to become new speedsters, while Barry continues being the Flash before his retirement.

On July 2, 2022, The Flash showrunner Eric Wallace revealed that the cast and crew did not expect the series to be renewed for a ninth season, so the writers wrote "Negative, Part Two", the season eight finale, as the series finale. That episode was originally set to end with a scene in which team Flash would bid farewell to each other, but when The CW renewed the show, Wallace and the writers removed the scene and replaced it with a teaser of Cobalt Blue.[8]

When the ninth season was greenlit, Wallace expected the network to renew the series for an additional tenth and final season. After it was decided that the show would end with a 13-episode ninth season, he thanked the network for notifying him a year in advance. Although The Flash was the last remaining active series of the Arrowverse, after Superman & Lois was retconned out of the continuity into another universe, Wallace did not write "A New World, Part Four" as an ending for the Arrowverse, hoping that the franchise would be continued through other shows.[9]

In preparation for writing the final episodes, Wallace rewatched some of his favorite series finales, taking inspiration from those of Angel, Lost and The Office. He said that he wanted to generate similar conclusive emotions to those in the Lost finale and drew parallels with The Office finale in featuring the returns of former main characters. He considered ending on a cliffhanger as in the Angel finale, but decided that such a dramatic plot device did not fit The Flash, promising a "properly" last episode with a just a small nod at the future.[10]

Bringing back Eobard Thawne / Reverse-Flash for the finale was always part of the show's plan, according to Wallace, who felt that Barry's story had to end with "one final confrontation" with his archenemy.[11] The episode also brings Hunter Zolomon / Zoom back to fight against Barry one last time, a reunion that Wallace had been hoping to write for several seasons.[12] Despite this neither Thawne nor Zolomon fight Barry instead being paired off with Allegra and Khione respectively.

The episodes include several callbacks to previous seasons, including Barry warning his past self not to intervene in his mother's murder,[lower-alpha 1][13] The finale moments of part four call back to the Pilot with a monologue mirroring the opening monologue from the first season as well as the chemicals floating when right before speed is granted to Avery Ho.[14] In addition, the "big bads" of seasons 1-3 and 7 also appear.[lower-alpha 2]

Main cast members Grant Gustin, Candice Patton, Danielle Panabaker, Danielle Nicolet, Kayla Compton, Brandon McKnight and Jon Cor appear as Barry Allen / Flash, Iris West, Caitlin Snow and Khione, Cecile Horton / Virtue, Allegra Garcia / Accelerant, Chester P. Runk and Mark Blaine / Chillblaine.[15]

The guest cast includes Matt Letscher and Tom Cavanagh, Jesse L. Martin, Rick Cosnett, Jessica Parker Kennedy, Teddy Sears, Karan Oberoi, Andre Tricoteux and Tobin Bell, John Wesley Shipp, Patrick Sabongui and Carmen Moore reprising their roles as Eobard Thawne / Reverse-Flash, Joe West, Nora West-Allen / XS, Hunter Zolomon / Zoom, August Heart / Godspeed, Barry Allen / Savitar, Jay Garrick / Flash, David Singh, and Kristen Kramer respectively. Cavanagh additionally reprises his role as Earth-1 Harrison Wells. Stephanie Iszak co-stars as Daisy Korber.[15][12] Cosnett's return as Cobalt Blue completely "blew away" Gustin,[16] admitting that the writers had always built moments to foreshadow a fan-rumored storyline.[9] The episode features several cameos from future speedsters including the previously introduced Avery Ho, portrayed by Piper Curda, as well as new characters Max Mercury and Jess Chambers, portrayed by Trevor Carroll and Destiny Huggins respectively.[17]

Carlos Valdes, who portrayed Cisco Ramon / Vibe in previous seasons, considered a return for the series finale so he could "round up" his run in the show since his first appearance in the pilot episode, having left in the seventh season in order to pursue new creative opportunities and deal with mental health issues. However, scheduling conflicts with his starring role as Miguel in the Hulu series Up Here prevented him from reprising the role. Ahead of the finale's airing, Valdes felt "heartbroken" at his failure to work out a reprisal as Cisco. Nonetheless, Valdes expressed his hopes that Cisco is happy now with Kamilla Hwang, feeling that his character deserves happiness.[18]

Wallace wanted Jordan Fisher to reprise his role as Bart Allen / Impulse in the finale; Fisher was unable to do so due to scheduling conflicts with his role in the Broadway musical Sweeney Todd.[19]

Preparation for the episode had begun by February 26, taking place in the same location of downtown Vancouver, Canada, where the fight scene between the Flash, Reverse-Flash and Godspeed took place in the seventh-season finale "Heart of the Matter, Part 2". Reports confirmed the return of several cast members, though noted that stunt doubles filled in to portray Godspeed and Savitar, while Gustin was replaced for some scenes by Cody Laudan due to not working a whole night of shooting.[15] Gustin contracted COVID-19 during filming six days before wrapping up the episode, so he had to be quarantined for ten days before returning to the set, resulting in the production shutting down one day before Gustin returned.[20] According to Panabaker, scenes were shot without Gustin to add him in later through green screen and one extra day of filming was ordered to enable him to return to the set.[9]

All four parts were first aired in the United States on The CW on a weekly basis from May 3, 2023, through to May 24. The episodes average about 0.43 million viewers per episode. All four parts held a 0.1 share among adults 18–49. Part two was the lowest viewed, with 390,000 viewers.[21] The second highest was part one with 420,000 viewers[22] The final two parts tied for first, both with 460,000 viewers.[23][24]

Charles E Henning of Tell Tale TV praised the choice to introduce a new villain and stated his excitement for the final three episodes.[25] CBR's Sam Stone said that while he enjoyed the episode, the humor fell flat and was cringeworthy at times.[26]

Nicole Drum of ComicBook.com speculated the transformation of Eddie Thawne into Colbalt Blue as the final big bad.[27]

Sam Stone of CBR praised Cosnett's performance as Eddie Thawne.[28]

Charles Raymond of Screen Rant found it upsetting how easily defeated the villains were, claiming that it rendered previous seasons seem pointless.[29]