The cover page to the upcoming, eighteenth episode of Supergirl's first season, titled "Worlds Finest," has been released online.
That's the episode which will guest star Grant Gustin as Barry Allen/The Flash, and will air on March 28.
As commenters have already noted, it seems likely the "Worlds" rather than "World's" is a clue that there is more than one Earth at play.
Fans have debated whether any potential crossover between The Flash and Supergirl would require travel between alternate earths -- a concept familiar to viewers of The Flash but new to Supergirl fans -- since there had been repeated references to Superman's fame and career on Supergirl and no such references on Arrow or The Flash.
World's Finest was a long-running comic book series that usually starred Batman and Superman together. More recently, when DC launched the New 52, they had a Power Girl/Huntress team-up book titled Worlds' Finest, the title of which was meant to reflect that while they were operating on the main DC earth, they came from Earth-2.
A photo posted by Grant Gustin (@grantgust) on
After months of rumors, CBS announced earlier today that Grant Gustin will appear on Supergirl.
"We are so incredibly excited to announce something that we have dreamed of happening since we starting making Supergirl – The Flash and Supergirl are teaming up!," said The Flash and Supergirl Executive Producers Greg Berlanti and Andrew Kreisberg. "We want to thank Grant Gustin for making the time to come visit (on top of his already immense workload) and all of the folks at CBS, The CW, Warner Bros. and DC for working this out. And finally thanks to the fans and journalists who have kept asking for this to happen. It is our pleasure and hope to create an episode worthy of everyone's enthusiasm and support."
Supergirl airs Monday nights at 8 p.m. ET/PT on CBS. The Flash airs Tuesdays at 8 p.m. ET/PT on The CW.