The episode sees Barry and his daughter Nora travel back in time and take a trip down memory lane where they cross paths with several of the show's past speedsters in order to gather some key necessities to stop Cicada.
Several recurring guest stars make appearances, including Michelle Harrison, John Wesley Shipp, Teddy Sears, and Tony Todd.
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.
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.
[2] 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.
[5] At the end of October 2018, Tony Todd, who voiced Zoom in the second season, revealed the episode would feature "all the [past] speedster villains" returning "wanting a piece of Barry".
[6][3] In mid-November 2018, The CW announced that the episode was titled "What's Past is Prologue", written by Helbing and Lauren Certo, and released the synopsis revealing that Barry and Nora would be travelling back in time to "gather some key necessities" to stop Cicada.
"[10] Main cast members Grant Gustin, Candice Patton, Danielle Panabaker, Carlos Valdes, Hartley Sawyer, Danielle Nicolet, Jessica Parker Kennedy, Chris Klein, Tom Cavanagh, and Jesse L. Martin appear in the episode as Barry Allen / Flash, Iris West-Allen, Caitlin Snow / Killer Frost, Cisco Ramon / Vibe, Ralph Dibny / Elongated Man, Cecile Horton, Nora West-Allen / XS, Orlin Dwyer / Cicada, Sherloque Wells and Joe West, respectively.
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.
[12] 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.
[15] A pre-credits scene teasing the events of crossover aired at the end of The Flash's "What's Past Is Prologue", Arrow's "Unmasked", and Supergirl's "Bunker Hill".
The website's critical consensus reads, "'What's Past is Prologue' takes viewers on a Flash-tastic journey back in time with a series of flashbacks that make for an eventful, charming, and commemorative 100th episode.
The time travel premise worked whenever the focus remained on Harrison Wells, but too much of this material played out as if Barry and Nora were watching a clip show of past seasons.