When Grant Gustin's Barry Allen, aka "The Flash," turns up on Supergirl for the first time, he won't just be continuing the universe-hopping storyline that has reverberated throughout his own series since the finale of the first season. He'll also be bringing the entire concept of the DC multiverse beyond the constraints of his fellow CW series (Arrow and Legends of Tomorrow) to an entirely different TV network and production team - potentially establishing precedent for what's being called The DC Extended Universe to one day share characters and storylines across even more disparate mediums and even feature films.
Now, in a new video clip from the forthcoming episode, the characters get an abbreviated explanation as to how the Multiverse actually works.
There's a certain symmetry to The Flash being the ambassador to the Multiverse, as the entire concept originated in the 1960s in the pages of the rebooted Flash comic books. In the story, Barry discovers the existence of an "Earth 2" where (slightly) older versions of Earth's DC heroes continue to exist in their Golden Age characterizations - and reasons that comic book authors from Earth 1 must have tapped into visions of Earth 2 in their dreams, unwittingly "borrowing" characters like Jay for their fiction.
In the Supergirl clip, we learn that the DC TV version of the Multiverse at least "works" the same basic way. Alternate realities are separated by vibrating on different universal frequencies, with Barry able to travel between them by manipulating the rate of his own molecular vibration using his speed force powers. But he also confirms that, unlike the comics, each universe is not necessarily populated by doppelgangers of the same people. Not only is there no (known) Flash on Supergirl's world, he reacts with surprise to learn that Supergirl herself is an alien - seemingly confirming that there is no Superman on his world either - at least, not that anyone knows of.
In the comics, the Multiverse eventually became a way for DC to experiment with radical changes to established characters and create "homes" for clusters of characters inherited from defunct comics publishers like Fawcett, whose Captain Marvel characters were relocated to "Earth S." Due to fears of reader confusion, DC collapsed the Multiverse into a singular continuity with Crisis on Infinite Earths in the 1980s. The concept has been revived in various formats over the years, but none as expansive or broadly-encompassing as the original version.
Of note, the characters of Flash and Supergirl are shown in the clip theorizing about the existence of even more Earths beyond their own, including one where JFK survived his assassination or another where "the Nazis won World War II." Longtime DC fans will likely raise an eyebrow for that second one, which suggests a reference to Earth-X - a pre-Crisis world where WWII continued into the (then) present, and home to patriotic heroes from National Comics like Uncle Sam and Phantom Lady.
The Supergirl and The Flash crossover episode, ‘Worlds Finest,’ airs next Monday @8pm on CBS.