Comics fans are understandably excited that Flash movie, but the concept likely wouldn't have happened had it not been for a single Flash comic that came out in the height of the Silver Age. The alternate reality adventures of the film, as well as seminal stories like Crisis on Infinite Earths would not be possible without this issue. It marked a major milestone for DC Comics with a groundbreaking story that has had ripple effects felt across the entire comics industry ever since.
The one of Barry Allen's favorite comic book characters, and upon discovering his new speed, names himself after the Flash from the comics. After a few adventures in Showcase, the Flash was given his own title again in 1959's The Flash #105, picking up the numbering of the old series before it was cancelled. This new Flash was popular with readers, but no one could have prepared for what was in store in 1961's The Flash #123.
With a prophetic cover blurb that read "Flash of Two Worlds! A Spectacular Story That Is Sure To Become a Classic!" this story was penned by the Golden Age Flash's original creator Gardner Fox, and would not only bring Jay Garrick back into comics, but it would also fundamentally change the genre. The story starts in a relatively mundane way, with Barry Allen performing feats of super-speed for a crowd of orphans, but when he attempts to try a new trick by vibrating fast enough to turn invisible, he accidentally vibrates into a parallel Earth. Barry begins to recognize names from the comics he used to read, and is surprised to meet Jay Garrick, now retired, who is very much flesh and blood. After taking out some of Jack Garrick's old villains together, Barry returns to his Earth by vibrating at the same frequency that got him there. He inspires the Golden Age Flash to come out of retirement, and in a single issue, introduced the world of comics to the Multiverse.
Some moments in the story are amazingly meta, with Barry saying the original Flash comics were written by Gardner Fox, the actual writer of The Flash and the writer of The Flash #123. Barry says Fox got his comic ideas from dreams and that his subconscious must have tapped into a parallel Earth while he slept, and proposes the possibility that Fox's comic stories are actual s from other dimensions. Barry then surmises that there must be a Flash for every alternate Earth that exists, of which there must be an infinite number. At the end of the issue, Barry says he'll have to track down Gardner Fox and tell him the story, amazingly implying the issue itself is a factual of dimension-hopping.
The ideas contained in this single issue opened the floodgates, introducing the concept of multiple Earths and providing the basis of almost every company-wide crossover ever since. With the DCEU fully embracing the Multiverse by reintroducing Michael Keaton's Batman into the Flash movie, it is echoing the sentiment of Gardner Fox's The Flash #123 by bringing back a fan favorite character through the magic of science fiction storytelling.
