CBS reveals the details for The Flash and Supergirl crossover episode “Worlds Finest”, along with a release of a comic styled promotional poster for the episode, which is a recreation of an old Superman vs The Flash comic book cover.In the article released by Variety, the plot summary for the episode is unveiled, along with how Barry ends up on Supergirl’s alternate universe, confirming that the Arrowverse doesn’t exist in the same universe as Supergirl’s:
Titular heroine Kara (Melissa Benoist) gain a new ally when the lightning-fast superhero The Flash, aka Barry Allen (Grant Gustin) suddenly appears from an alternate universe. Barry helps Kara battle Silver Banshee, aka Siobhan Smythe (Italia Ricci), and Livewire (Brit Morgan) in exchange for her help in finding a way to return him home.
The Scarlet Speedster already has some experience with alternate universes, having paid a visit to Earth-2 — another version of our world, albeit with some striking differences — in Season 2 of the CW series. During that trip, “The Flash” teed up the impending crossover by including a shot of Benoist’s Supergirl while Barry traveled through the breach between worlds, hinting at a larger DC Comics multiverse that offered endless possibilities on infinite Earths.
While Supergirl and Superman don’t exist on Barry’s Earth-1 — which is set in a continuity known as the “Arrowverse,” since it encompasses all of the superhero shows spawned by The CW’s “Arrow,” including “The Flash,” “DC’s Legends of Tomorrow” and animated series “Vixen” — Barry’s ability to travel through time and space means that he’s capable of paying a visit to other DC Comics-based shows (even on other networks) without disrupting anyone’s reality.