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. Production Development and writing At the 2018 San Diego Comic-Con, it was announced that the eighth episode of The Flashs fifth season and the series' 100th episode would not be taking place during the annual Arrowverse crossover event like had been the case with the previous seasons' eighth episodes. The Arrowverse showrunners "flipflopped" the placement of the crossover in order to make the milestone 100th episode separate, with the ninth episode being part of the crossover instead. Cast member Tom Cavanagh was also revealed to be directing the episode. Executive producer and showrunner Todd Helbing said the episode will act as a sort of midseason finale and be "a little bit different," with "a big twist to carry us over". Ahead of the premiere of the fifth season, Helbing explained that "[they] wanted to do something special" for the episode by "taking the audience on a trip to remind everybody of all the cool stuff we've done for five years now. We wanted to relive the coolest moments on our show, so we came up this concept and it both honors what we've done so far and also propels the show for the rest of the season." Cast member Candice Patton also commented that the