It's time for an epic gathering of superheroes on the final hour of the four-part "Crisis on Earth-X" DCTV crossover which concludes on Legends of Tomorrow. Over 20 superheroes join Green Arrow, The Flash, Supergirl and White Canary to take down the Earth-X villains.
The official episode description from the CW says, "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 (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 (Stephen Amell), Supergirl (guest star Melissa Benoist), The Flash (guest star Grant Gustin) and White Canary (guest star Caity Lotz) -- lead their teams into battle to save the world."