Felicity and Iris are captured by Dark Arrow and Dark Flash. With the team save them in time? Oliver and Barry enlist the help of Citizen Cold, The Ray and their friends to find a way to protect their loves. The DCTV four-part crossover, "Crisis on Earth-X" continues on The Flash.
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."