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. Two weeks later,
Jessica Parker Kennedy revealed that in the episode Nora gets to see a special moment between her mom and dad "that like really breaks her heart". Patton added, "She gets to see Barry and Iris as we've known to love them over the past four seasons," with Parker Kennedy concluding that "the episode really brings the three of them closer." Cavanagh also plays Harry Wells of
Earth-2 and Eobard Thawne / Reverse-Flash. Martin appears in the episode only in archived footage due to a back injury he sustained prior to the start of filming of the season. When interviewed by
Entertainment Weekly after the episode's release, Helbing revealed that early versions of the episode had intended to see
Rick Cosnett reprising his
first season role as
Eddie Thawne, but scheduling conflicts with the episode's tight filming schedule prevented Cosnett from returning. Helbing also hinted that, had it not been for potential scheduling conflicts, the crew would have liked to bring
Tom Felton and
Shantel VanSanten back as
Julian Albert / Doctor Alchemy and
Patty Spivot respectively, in addition to possibly having a longer scene for
Teddy Sears as Zoom.
LaMonica Garrett as
Mar Novu / Monitor and
John Wesley Shipp as Barry Allen / Flash from
the 1990 television series appear in a
pre-credits scene. Shipp also briefly appears as
Henry Allen.
Filming Filming of the episode began on September 26, 2018, and concluded by early October. Gustin thought it was great that they were able to have Cavanagh direct the episode, saying "[Cavanagh] took such good care; he understands the journey... we've found a lot of details that probably wouldn't have been found otherwise."
Arrowverse tie-ins In November 2018, it was revealed that the eighth episodes of
The Flash,
Arrow, and
Supergirl would feature "puzzling events" that lead to the events of the annual Arrowverse crossover event, titled "
Elseworlds". A pre-credits scene teasing the events of crossover aired at the end of
The Flashs "What's Past Is Prologue",
Arrows "
Unmasked", and
Supergirls "
Bunker Hill". The scene also confirmed Earth-90 as the earth for the Flash from
the 1990 television series.
Marc Guggenheim, who acted as the showrunner for the crossover, felt the tease was "cool because we've never... setup the crossover in the previous episodes before." The tease was meant to be one of two options for the "Elseworlds"
cold opening. The writers could not choose which they wanted to use, with Guggenheim explaining someone decided to "take one cold open, move that and make that the post-credit tag, like the way
The Flash typically does, at the end of all three shows, and then have the other cold open at the beginning of hour one."
Supergirl co-showrunner
Robert Rovner added that the tease was only supposed to air with
The Flashs eighth episode, before being added to the other two episodes to make things easier for viewers since they "are coming to [the crossover] from our separate shows and we wanted something that [connects them together]." ==Release==