Plot summary
Barry Allen and Nora West-Allen travel back in time to gather several items to create a power dampener to stop Cicada: a highly magnetic alloy from Savitar 's armor, Harry Wells ' Speed Force transmitter, and dark matter. They first travel to Barry's final battle against Savitar. Despite the intervention of a Time Wraith, they manage to retrieve a piece of the armor. They next travel to when Hunter Zolomon stole Barry's speed with the transmitter, which Barry obtains from Harry. As Barry and Nora attempt to travel back to the night of the particle accelerator explosion, Zolomon discovers them and gives chase before being stopped by a Time Wraith, causing Barry and Nora to land on the day that Barry's 2016 self went back in order to get faster instead.
Nora convinces a reluctant Barry to ask Eobard Thawne for help in repairing the transmitter, which was damaged during their landing. Thawne ultimately fixes the transmitter after Barry threatens to disrupt his plans. Barry and Nora travel to the night that the particle accelerator exploded to collect the dark matter released and complete the power dampener. Barry reveals to Nora that Thawne killed his mother, and they subsequently hide the dampener inside a hospital's pillar before returning to the present so they can confront Cicada there. Cisco Ramon separates Cicada from his dagger with the dampener only for the latter to re-summon it. As Cicada prepares to kill Barry, Caitlin Snow transforms into Killer Frost and subdues him, forcing him to flee.
Meanwhile, Sherloque Wells continues to investigate Nora's true intentions for being in 2018 by translating the strange symbols in her journal, which Nora claims that she created in order to keep track of changes to the timeline. Sherloque returns the journal but hides an already deciphered message that reads, "The timeline is malleable." Later, Nora travels to Iron Heights Penitentiary in 2049 to confront Thawne, to whom she has secretly been sending her journal entries.
In a pre-credits scene, the Flash of Earth-90 fails to stop a mysterious individual from using a powerful book to destroy his universe and is forced to run.
Credits
Adapted from Wikipedia contributors, What's Past Is Prologue (The Flash). Paragraphs and formatting adjusted; illustrations, reference markers, block quotations and non-story material omitted.