Barry and Iris's 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, The Ray, Felicity Smoak, Iris West and Alex Danvers to take on their most formidable villains yet. Earth's mightiest heroes - Green Arrow, Supergirl, The Flash and White Canary - lead their teams into battle to save the world.

The crossover starts on Supergirl S03E08 Crisis on Earth-X (I), Arrow S06E08 Crisis on Earth-X (II) and The Flash S04E08 Crisis on Earth-X (III).

Rattled by recent events, the Legends dive into work which finds them investigating an Anachronism in a Viking settlement in the New World. The Legends realize that the Norsemen are worshiping an artifact as their god and are surprised by the artifact's origin. For the first time, Sara is worried that they might need back up when Damien Darhk shows up. Meanwhile, Jax finds a loophole that could potentially change history, but it is a risk he is willing to take.

After Sara's encounter with Mallus, the Legends are paid a visit by John Constantine, a demonologist detective. The Legends agree to accompany him to a present-day psychiatric hospital and they are surprised to discover who Constantine is trying to help. During the exorcism, Sara, Leo and Constantine go missing, leaving Ray and Zari to try to take care of Constantine's client. Meanwhile, Amaya and Nate once again come face to face with Kuasa.