SPOILERS Below For Crisis On Earth-X!
Crisis On Earth-X adds a new twist to the DC Comics Multiverse of 52 parallel Earths. The CW's Arrowverse series The Flash, Arrow and DC's Legends of Tomorrow exist on what's known as Earth-1, while Supergirl takes place on Earth-38. Though there are differences between those two Earths, they resemble our own real world, albeit their worlds boast superheroes and super villains. Crisis On Earth-X introduces the heretofore unheard of 53rd Earth in the Multiverse - Earth-X! The difference: Earth-X is controlled by Nazis.
According to the Harrison Wells of Earth-2 (Tom Cavanagh), Earth-X doesn't have a numerical designation (from whomever grants the 52 Earths their assigned numbers) because Earth-X is "so awful, so horrific that no sane person would ever travel there." Earth-X is basically the same Earth as Earth-1, with the same history and geography until the Second World War. That's the crucial divergence point. On Earth-X, the Nazis developed the atomic bomb before the United States and the Allies did. With their atomic weapons, New York, London, Paris, and Moscow were "all obliterated" (though Star City and Central City were spared). The Nazis won the war and captured the world.
When the war was over, Adolph Hitler ruled the world until his death in 1994. The S.S. (Schutzstaffel) set up outposts throughout Europe and the Americas, while the Third Reich was endorsed by the Prime Ministers and Presidents of the defeated nations who were sympathetic to the Fuhrer. All of Earth-X was united as one 'Fatherland'.
Someone who grew up in Earth-X is the Nazi doppelganger of Tommy Merlyn (Colin Donnell), one of the Arrowverse crossover's many surprises. Captured by our heroes and imprisoned in the Pipeline underneath STAR Labs after the Nazis attacked the wedding of Barry Allen (Grant Gustin) and Iris West (Candice Patton), Tommy confessed to Earth-1's Oliver Queen (Stephen Amell) that to be born into the Reich means the Fatherland is your whole world. Growing up in Earth-X means attending mandatory youth groups and military training designed to ferret out the weak.
Just as there are 53 Earths, it also means there are 53 Kryptons. The Krypton in Earth-X's universe also exploded and a space craft also brought Kara Zor-El (Melissa Benoist) to Earth-X. Landing in the Fatherland, Kara was indoctrinated into the Reich - and unlike Earth-38's Kara, Kara-X's sister tried to kill her. However, Kara is the most powerful being on Earth-X and became both Overgirl and a general of the Reich. Overgirl in turn is one of the main reasons Earth-X invaded Earth-1.
Kara-X married the Oliver Queen of Earth-X, known as the Dark Arrow and the Fuehrer. They form a villainous triumvirate with Eobard Thawne, the Reverse-Flash of Earth-X (the fate of the Flash of Earth-X has yet to be revealed). Kara-X is dying, however. Her cells have absorbed too much yellow sun radiation. The Nazis stole a prism from Dayton Labs that can duplicate red sun radiation. With the prism, they plan to cut into Supergirl in order to give Overgirl a heart transplant.
As bleak as Earth-X is, with its prison camps and Nazi banners flying over every city, there are pockets of hope. There are freedom fighters trying to stop the Nazis. One of them was James Olsen (Mehcad Brooks), AKA the Guardian, whose shield is adorned with the American flag that hasn't flown in Earth-X since 1945. The Dark Arrow killed Olsen when Crisis on Earth-X began, but Olsen's dying breath promised that there are others like him.
No doubt there will be even more surprises, and hopefully even more background on the history and cultural makeup of Earth-X, to come when the second half of Crisis On Earth-X unfolds on The Flash and Legends of Tomorrow.
Crisis On Earth-X concludes on The Flash @ 8pm ET and DC's Legends of Tomorrow @ 9pm ET on The CW.