The Flash’s first season was defined by its admirable embrace of some of the Flash’s strangest comic book concepts: the mystery of the Reverse Flash, time travel, the goofy Rogues, and yes, Gorilla Grodd. But its second season is tackling something altogether crazier—the DC comics multiverse. Here’s what you need to know going in.

Welcome to Earth

In the beginning, Earth-Two wasn’t Earth-Two: it was just Earth. Back in the 1940s, comics were in what was called their “Golden Age,” the superhero comic boom that saw the birth of iconic heroes like Batman, Superman and Wonder Woman.

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But as the 1950s came along, the superhero fad died down, and comic books moved on to other trends, like westerns and horror anthologies. DC ultimately scrapped many of its superhero comics, and only what is now referred to as their “Trinity” (Wonder Woman, Batman, and Superman) made it out of the ‘40s in one piece.

By the 1960s, superheroes had become a dominant force in comic books again, and DC felt reenergized to break out their roster of heroes introduced in the ‘40s, but with a twist. The magic and mysticism that provided the basis for many heroes like Hawkman and Green Lantern was erased, replaced by new origins rooted in science fiction. DC’s bizarre reboot didn’t just change origins, it changed so much that this new roster of heroes was similar to their Golden Age counterparts in name and power only. Now Green Lantern was a space cop named Hal Jordan instead of Alan Scott. Carter Hall’s human Hawkman became a Thanagarian alien named Katar Hol. And most important to us, the first Flash, Jay Garrick, was replaced by Barry Allen.

The Flash of Two Worlds

These new incarnations, alongside the ever-present Batman, Superman and Wonder Woman, got their own comic series and eventually teamed up to form the Justice League of America. But shortly after the superhero revival, DC decided to have their old characters meet up with their new incarnations: and they did so by establishing a whole new parallel universe, where the adventures of the original 1940s DC superheroes never finished as they did in our own world.

The concept, which created “Earth-One” (the home of the Silver Age heroes) and “Earth-Two” (the home of the Golden Age heroes), made its first appearance in the now-iconic Flash #123, in a story called “Flash of Two Worlds.” Original Flash Jay Garrick teamed up with then-current Flash Barry Allen after Barry ran so fast, he essentially managed to travel between dimensions and land on an Earth similar to his own, but with a whole different cast of superheroes.

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“Flash of Two Worlds” opened the floodgates for Earth-One and Earth-Two characters to begin interacting with each other. More and more Golden Age characters didn’t just return, but operated openly alongside their modern counterparts in what would eventually form into DC’s Multiverse: an interconnected weave of parallel Earths. The Justice League and Justice Society met up in annual comics that proved to be incredibly popular. DC revived All-Star Comics to tell new stories with Golden Age characters. Certain popular alternates, like the Superman of Earth-Two—dubbed “Kal-L”, the original spelling of his Kryptonian name of Kal-El, to differentiate from the Silver Age Superman—got their own stories in the long-running monthly anthology series Superman Family. The alternate Earth became more than a one-off storytelling trick, it became everything that DC’s superhero comics were about.

Bye Bye, Earth-Two

But 20 years after “Flash of Two Worlds” changed everything for DC, the multiverse was considered a mess by writers. The overlong continuity and confusion at having not just alternate versions of popular characters, but having them constantly interacting with each other, was seen as both a burden to telling new stories and alienating to new readers. Thanks to having two different origins, no character had a reliable, or easy-to-understand backstory. So for their 50th anniversary, DC planned to wipe the slate clean once and for all: and Earth-Two had to go.

Beginning in 1985, Crisis on Infinite Earths was a huge comic crossover series that ultimately saw DC’s multiverse merged into a single New Earth. Every character was essentially rebooted into a new continuity, and while Earth-2 was gone, several characters, such as Jay Garrick, and even Power Girl, were kept as parts of the new world, relics from a world now gone. Although follow-up “Crises” events would deal with the threatening return of the DC multiverse, for now, Earth-Two and many of its heroes were no more.

Hello Again, Earth-2

It would be another 20 years for DC’s multiverse to make a true return. Following the events of the 2006 arc Infinite Crisis, which saw Lex Luthor attempt to recreate and the merge Earth-Two with the New Earth, the weekly comic series 52 revealed that 52 alternate Earths, copies of the New Earth all altered in subtle ways, still existed—including Earth-Two. But although this recreated Earth-Two had many similarities to the Earth-Two established in “Flash of Two Worlds” it was actually considered to be a separate creation altogether (revealed in a 2008 arc where Power Girl traveled to the second Earth-Two, believing it to be her lost home).

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The alternate-alternate Earth was, unsurprisingly, seen as far too confusing for DC to continue with, and when they wiped the slate clean again for their 2011 reboot “New 52,” another new version of Earth-Two appeared: but just like the main DC comics universe, this one had also been rebooted altogether. There was one new catch: the “main” DC universe was cut off from all other realities, including the new Earth-2, in an attempt to avoid the mistakes of the original intermingling seen in 1960s.

Renamed “Earth-2” and covered in the series Earth-2 and World’s Finest, Earth-2 featured many Golden Age versions of DC’s heroes, but supplanted into a more modern world where superheroes had only just began to emerge. Batman, Superman, and Wonder Woman all sacrificed themselves on this Earth-2, leaving the re-emergence of characters like Jay Garrick, Alan Scott, and Kendra Saunders as Hawkgirl to form a new Justice Society of America. This Earth-2 faced a series of apocalyptic events that nearly saw it destroyed for a third time: before this year’s Convergence event re-connected the DC Multiverse and saved it once more, once again leaving Earth-2 as an alternate world, filled with opportunities for alternate stories, for DC to fall back on.

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But now that Jay Garrick is set to appear on TV in this week’s episode of The Flash, it looks like we could see a whole new incarnation of Earth-2 ushered into DC’s long history—one that, just like the different alternate Earths of DC past, is slightly different to the one we’re all familiar with.