Kara and Alex travel to Earth-1 to attend the much-anticipated wedding of Barry Allen and Iris West, where they cannot get too far from their own relationship issues. But when an unknown group of criminals attack the festivities, they must join forces with Earth’s mightiest heroes.

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" BEGINS

Barry (guest star Grant Gustin) and Iris's (guest star 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 (Chyler Leigh) to take on their most formidable villains yet. Earth's mightiest heroes - Green Arrow (guest star Stephen Amell), Supergirl (Melissa Benoist), The Flash (guest star Grant Gustin) and White Canary (guest star Caity Lotz) - lead their teams into battle to save the world.

Larry Teng directed the episode with story by Andrew Kreisberg & Marc Guggenheim and teleplay by Robert Rovner & Jessica Queller (#308).