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 continues on Arrow S06E08 Crisis on Earth-X (II), The Flash S04E08 Crisis on Earth-X (III) and DC's Legends of Tomorrow S03E08 Crisis on Earth-X (IV).

Supergirl investigates a mysterious symbol popping up all over National City, tracing its origins back to an ancient prophecy and the mark of the World Killer, Reign. Meanwhile, the tension between Lena and Morgan Edge continues to build, causing James to step in and offer up some protection, and possibly igniting a spark in the process.

Struggling to heal from her injuries inflicted by Reign, Supergirl remains in a dreamlike state, unable to be reached. Mon-El recruits one of the Legion members, Brainiac-5, to try to bring her back. Meanwhile, Reign continues her rampage on the city so the DEO teams up with the Legion of Superheroes to try to stop her.

As Reign continues to terrorize the city, Supergirl teams up with Saturn Girl, and the two recruit Supergirl’s former enemies, Livewire and Psi, to form a team for a mission to Fort Rozz to recover a prisoner who has information on how to defeat Reign once and for all. Meanwhile, Alex babysits Ruby.

Someone tries to kill Morgan Edge and he barely survives. He accuses Lena of being behind the attempt and vows to destroy her. When Lena fears her hatred for Morgan will take her to the Luthor dark side, Kara steps in to help her friend. Meanwhile, after Sam realizes she’s losing time, she confides in Alex who decides to run some medical tests to see if she can find out what’s going on with her new friend.

The DEO captures a second Worldkiller, Purity. Supergirl and Alex use very different methods of interrogation with Purity in the hopes of finding out how to defeat Reign. However, Purity escapes and attacks the city’s subway tunnels. Meanwhile, J’onn, Winn and Mon-El band together to fix the Legion ship.

When Winn’s father dies, his estranged mother, Mary, reappears trying to reconnect and to explain why she abandoned him all those years ago. Winn is too embittered from his rough childhood to forgive her but when a copycat Toyman attacks, the two must work together to stop him. Mon-El tells Kara something important about the Worldkillers, and Alex becomes suspicious of Myr'nn’s recent behavior.

Myr'nn inadvertently causes psychic disturbances at the DEO; Supergirl must work with J'onn to contain the chaos; Mon-El begins training Kara in fighting techniques he's learned in the future.

Supergirl and Imra have different ideas on how to stop the third Worldkiller, Pestilence. However, when both Alex and Winn are seriously injured by Pestilence, Supergirl and Imra team up to stop the destruction.

Still reeling after learning about Lena’s secret, Supergirl asks Guardian for a favor that could alter his relationship with Lena forever. Meanwhile, with all three Worldkillers united, Supergirl and the team brace for an epic battle.

When Reign starts hunting Ruby, Supergirl and Alex work together to keep her safe. Supergirl and J'onn look to Sam’s mother for advice on how to stop Reign and come away with some shocking news.

When Tanya, a disciple of Coville's, escapes from what's left of his cult, she gives Kara and James a journal that could hold the key to saving Sam. Someone learns Guardian's identity and threatens to expose James if he doesn't do what they ask. James contemplates what it means to be a black vigilante without a mask in America.

Supergirl is stunned to discover that a part of Krypton survived. Meanwhile, Alex is attacked while out with Ruby, and Lena considers how far she will go to keep Reign contained.

Kara makes a major life decision. Meanwhile, J'onn finds out that special DEO-caliber guns have hit the streets of National City.

Supergirl learns the true depth of Serena's nefarious plans for Earth. Supergirl, Mon-El and Alura must devise a plan to stop her before Serena gets to Earth. J'onn prepares to say goodbye to his father.

Supergirl and team take on Selena in an epic battle for Earth.

Diving back into reporting, Kara welcomes a new cub reporter to CatCo. Meanwhile, James and Lena argue about James’ impending indictment for acting as the vigilante Guardian, while Alex and Brainy struggle to get in sync at the DEO. Supergirl is called into action when remnants of the anti-alien terrorist organization Cadmus try to assassinate pro-alien leaders, but their endgame turns out to be much more sinister than she expected. J’onn relishes his peaceful new life, but an outing with an old friend causes him concern.

A shocking revelation causes chaos in National City. Supergirl sets out to capture Mercy Graves, while Alex takes control at the DEO. Back at CatCo, Kara decides to write an investigative story on Mercy and asks Lena if she can interview her as Mercy has ties to the Luthors. Meanwhile, Brainy meets Nia and the two end up in a precarious situation.

Ben Lockwood didn't always hate aliens. Through a series of incidents, and input from his anti-alien father, Ben slowly transforms from a mild-mannered professor into the villainous Agent Liberty of today.

When Supergirl needs help, Alex asks Lena and Brainiac to team up. Meanwhile, J’onn questions his decision to quit the DEO. However, after running into Manchester Black, he realizes there are a lot of ways to help his fellow aliens during this tumultuous time.

Colonel Haley makes a surprising decision regarding Supergirl. Kara writes a series of articles highlighting aliens in National City in the hopes that humans will stop being so fearful of them. Unfortunately, the article puts some of the aliens in harm’s way.

Everyone gathers for Thanksgiving but Kara is feeling down after her televised debate with Ben Lockwood about the anti-alien sentiment coursing through National City. Meanwhile, James and Lena argue about the best way to handle the Children of Liberty.

James falls in deeper with the Children of Liberty in his efforts to meet Agent Liberty. Meanwhile, Supergirl and Manchester Black follow a lead on Agent Liberty’s location, but things take a dark turn. Lena kicks off her first set of trials.

Nia has a powerful dream about Agent Liberty but refuses to look at it as a prophetic dream and pushes it aside. After noticing something is bothering Nia, Kara enlists Brainy's help, and the two try to persuade Nia to embrace her destiny. Meanwhile, Manchester Black pays Ben Lockwood a menacing visit.

Supergirl, The Flash, Green Arrow and Superman engage in the battle of their lives.

The crossover starts on The Flash S05E09 Elseworlds (I) and Arrow S07E09 Elseworlds (II).

Colonel Haley is upset after Supergirl responds to a distress call from a Navy ship that turned out to be a deadly alien attack. Colonel Haley reclaims the site as DEO territory and demands that Supergirl step aside. After Supergirl refuses to be told what to do, Haley refocuses her energy on finding out Supergirl's true identity and begins to interrogate all DEO agents, including Alex. Meanwhile, Brainiac-5 tries to persuade Nia to embrace her superpowers but she misinterprets his motives.

Kara joins Nia on a trip to Nia's hometown to visit her family during the town's annual Harvest Festival. While home, Nia's mother encourages her daughter to embrace her destiny. Meanwhile, Alex deals with a street drug that is turning people violent and giving them temporary superpowers.

With his P.I. shingle newly hung outside his office, J'onn welcomes new clients. Trying to take her mind off what happened at the DEO, Kara decides to team up with J'onn on his latest assignment, which unfortunately ends up tying directly into Alex's current investigation that involves the villain Menagerie. Meanwhile, Lena shares some news with James, but his reaction isn't what she expected. Nia invites Brainy to her Valentine's Day party.

Manchester Black breaks out of prison with the help of his new team, The Elite, who are set on punishing the anti-alien forces. Supergirl pulls double duty as she tries to apprehend Manchester Black and his team as well as deal with a shocking new development with Ben Lockwood.

Between Ben Lockwood’s new push to stir up the anti-alien movement and the Elite’s desire to target Lockwood and his minions, Supergirl is concerned about the safety of the American people – both human and alien. When Lockwood organizes a rally, the aliens decide to peacefully protest. Brainiac and J’onn join the alien march while Supergirl and Dreamer patrol to keep everyone safe. Meanwhile, James picks up his camera again to cover the march for CatCo, and Hayley assigns Alex a job that clashes with her beliefs. When The Elite and Ben Lockwood stir up trouble at the rally, Supergirl is forced to take a stand.

Secretly furloughed from prison due to failing health, Lex Luthor visits his sister, Lena, to seek her help finding a cure. Savvy as ever, Lena is suspicious of Lex's motives but when she's faced with a life and death situation, she must decide how she truly feels about her brother. Meanwhile, Supergirl and J'onn face off against Manchester Black. James' sister Kelly comes to town.

In the wake of Lex Luthor's return, the show flashes back to what he's been doing for the last two years and how his secret machinations and plans have affected Supergirl and Lena.