'Supergirl' season 3, episode 8 synopsis: " Crisis on Earth-X, Part 1 "

This episode begins a crossover event that continues in Arrow season 6 episode 8, and concludes in The Flash season 4 episode 8 and Legends of Tomorrow season 3 episode 8.

Episode released on November 27, 2017.

Overview of the episode:
In a parallel world, Earth-X, Nazi regime rule the planet, where a dark hooded archer, Dark Arrow, personally hunts the American loyalists and other resistances. Kara and Alex arrive to Earth-1 for Barry Allen's wedding with Iris West, leading Alex to meet Oliver Queen's (Green Arrow) vigilante associate Sara Lance (White Canary), taking the first step to recover from her break up with Maggie. During the wedding ceremony, the event is interrupted by Earth-X invaders, led by Dark Arrow and his Kryptonian-powered wife Overgirl. After Kara injured Overgirl during the fight, the Nazis are forced to retreat, and the heroes have captured Dark Arrow's partner Prometheus. As Dark Arrow, Overgirl, and a yellow speedster regroup and discuss their next step, Dark Arrow and Overgirl are unmasked to be parallel universe doppelgängers of Oliver and Kara, and the yellow speedster is Barry's nemesis from the future Eobard Thawne, the Reverse-Flash.