"What's Past Is Prologue" is the eighth episode of the fifth season of the American television series The Flash, based on the DC Comics character Barry Allen / Flash, a crime scene investigator who gains super-human speed, which he uses to fight criminals, including others who have also gained superhuman abilities. It is set in the Arrowverse, sharing continuity with the other television series of the universe, and is a spin-off of Arrow. The episode was written by Todd Helbing and Lauren Certo, directed by Tom Cavanagh, and is the series' 100th episode. Grant Gustin stars as Barry, and is joined by principal cast members Candice Patton, Danielle Panabaker, Carlos Valdes, Hartley Sawyer, Danielle Nicolet, Jessica Parker Kennedy, Chris Klein, Cavanagh, and Jesse L. Martin. 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. Shipp also reprises his role as Barry Allen / Flash from the 1990 television series in a pre-credits scene that ties-in with the Arrowverse crossover "Elseworlds". "What's Past Is Prologue" originally aired on The CW on December 4, 2018, and was watched by 1.78 million viewers according to Nielsen Media Research. The episode received a positive critical response. Plot 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