Former series regulars including Tom Cavanagh, Jesse L. Martin, Rick Cosnett, and Jessica Parker Kennedy return for guest appearances.

"A New World" the first and fourth parts were co-written by showrunner Eric Wallace with Thomas Pound and Sam Chalsen respectively.

The possessed Joe then attacks him, stating that he is not the new Negative Speed Force avatar but still wants to reduce Barry's impact on the timeline.

After he gasses CCC Media, Khione rescues Taylor and Aariz by growing plant cells in their bodies, but their skin turns green.

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.

Now a speedster known as Cobalt Blue, Eddie resurrects Eobard Thawne, Hunter Zolomon / Zoom, Savitar and August Heart / Godspeed and gathers them in the Negative Speed Force.

The next morning, Nora is born and Harrison Wells tells Khione to ascend as the natural order's protector.

Barry unleashes a lightning bolt and chooses Avery Ho, Max Mercury and Jess Chambers to become new speedsters.

[8] 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.

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] Gustin envisioned possibly ending the series with Barry Allen / The Flash sacrificing himself to keep the Speed Force alive by time traveling back to the show's first episode and becoming the lightning that struck his past self and gave him his powers, thus closing a time loop and giving him a "hero's death" like a fan theory he read claimed, but Wallace opposed Gustin due to wishing a "happy ending" for Barry and Iris West-Allen in addition to leaving their future open.

[11] Retrospectively, Gustin was satisfied with the chosen ending due to Barry and Iris deserving happiness after being a consistent couple and was glad the showrunners didn't go ahead with his suggestion, as having Barry sacrifice himself despite Oliver Queen / Green Arrow / Spectre doing the same for him and their fellow heroes in "Crisis on Infinite Earths" would have "taken away" the impact of Queen's sacrifice.

[10] Other ideas considered for The Flash finale included an "emotional" and "bigger" one involving the Negative Speed Force that Wallace had conceived years ago, but didn't utilize due to the show not being renewed for a tenth season as he expected, and thus not giving enough time to set it up.

[12] 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.

[c][19] 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-Allen, Caitlin Snow and Khione, Cecile Horton / Virtue, Allegra Garcia / Accelerant, Chester P. Runk and Mark Blaine / Chillblaine.

[21][12] Additionally various actors who recurred in past seasons return including Teddy Sears, Karan Oberoi, Patrick Sabongui and Carmen Moore reprising their roles as Hunter Zolomon / Zoom,[13] August Heart / Godspeed,[23] David Singh, and Kristen Kramer respectively.

[28][29] Cosnett's return as Cobalt Blue completely "blew away" Gustin,[30] admitting that the writers had always built moments to foreshadow a fan-rumored storyline.

[31] Carlos Valdes, who portrayed Cisco Ramon / Vibe in previous seasons, considered a return for the series finale so he could "round up" his run on 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.

[33] Stephen Amell, who reprised his role as Oliver Queen / Green Arrow / The Spectre in the ninth episode, stated that when he was approached about appearing in the season, he specifically requested that his return not to be in the finale.

[42] Writing for Den of Geek Lacy Baugher praised the return of the various characters particularly that of Cavanagh's Harrison Wells.

[43] Comic Books Resources's Joshua Patton felt that the episodes provided closer and "closed the loop" of Barry Allen's origin story.

[44] Also writing for CBR Sam Stone felt that while he enjoyed the episodes, the humor fell flat and was cringe worthy at times, he praised Cosnett's performance as Eddie Thawne.

Charles Raymond of Screen Rant criticized the final battle, describing it as upsetting how easily defeated the villains were, claiming that it retrospectively caused the first and third seasons of the show to appear to have been pointless.