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 "What's Past is Prologue" was first aired in the United States on The CW on December 4, 2018. It became available for streaming on Netflix in Canada on December 6, 2018, while it was first aired in the United Kingdom on Sky One on December 6, as well. Reception Ratings In the United States, the episode received a 0.7/3 percent share among adults between the ages of 18 and 49, meaning that it was seen by 0.7 percent of all households, and 3 percent of all of those watching television at the time of the broadcast. It was watched by 1.78 million viewers. The episode improved 17% in the 18–49 rating on the previous episode, "O Come, All Ye Thankful". The episode attracted a total of 644,700 viewers within seven-days of its British premiere, making it the most watched programme on Sky One for the week. Critical response The review aggregator website Rotten Tomatoes reported a 93% approval rating for the episode, based on 14 reviews, with an average rating of 8.31/10. The website's critical consensus reads,