Supergirl finds herself captive of the invaders from a dark, parallel Earth, who wish to take her heart to save the life of one of their own. Meanwhile Oliver, Barry, and Earth’s other heroes must escape a Nazi camp on Earth-X — with help from that planet’s own heroes Ray and Citizen Cold.

U.S. air dates are in MM.DD.YY form. U.K. air dates are in DD.MM.YY form.

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THE EPIC FOUR-WAY CROSSOVER WITH "ARROW," "SUPERGIRL," "THE FLASH" AND "DC'S LEGENDS OF TOMORROW" CONTINUES

Barry (Grant Gustin) and Iris's (Candice Patton) wedding brings the gang together, but things go awry when villains from Earth-X attack the ceremony. All of the superheroes band together with help from their super friends like Citizen Cold (guest star Wentworth Miller), The Ray (guest star Russell Tovey), Felicity Smoak (guest star Emily Bett Rickards), Iris West and Alex Danvers (guest star Chyler Leigh) to take on their most formidable villains yet. Earth's mightiest heroes - Green Arrow (guest star Stephen Amell), Supergirl (guest star Melissa Benoist), The Flash (Grant Gustin) and White Canary (guest star Caity Lotz) - lead their teams into battle to save the world.

Dermott Downs directed the episode with story by Andrew Kreisberg & Marc Guggenheim and teleplay by Todd Helbing (#408).