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).

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.

Lex Luthor descends upon Washington, DC and summons Lena and Lillian Luthor to the White House. Supergirl realizes she has one last chance to stop Lex and turns to the power of the press to help her.

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.

Kara comes face to face with Red Daughter and the two engage in an epic battle. Determined to find a way to extract the Harun-El, Lena turns to an unlikely source for help - Lillian Luthor. Meanwhile, Alex realizes she's missing parts of her memory, and Brainy, Nia and J'onn set out to track down aliens who have been abducted by Lex.

Kara and Alex head home to Midvale for a little R&R. While there, Kara recalls a painful memory of a childhood friend's death and how that traumatic experience brought the Danver sisters closer together.

When multiple children get sick from lead poisoning, Morgan Edge points the finger at Lena and blames her creation of the lead bomb she made to save National City from the Daxamites. While Lena knows she never intended any harm, she fears there may have been a flaw in her design, leaving her responsible. Kara teams up with Samantha to clear Lena’s name, but it turns out to be harder than they thought. Meanwhile, Alex and Maggie make a crucial decision about the future of their relationship.

Supergirl must deal with the destructive aftermath of Lex's nefarious plans. Shocked at what Lex has created, Supergirl faces her biggest challenge yet.

When a new threat emerges in National City, Kara/Supergirl teams up with her cousin, Clark Kent/Superman, to stop it. Kara is thrilled to have family in town but it leaves Alex feeling a bit left out. Meanwhile, Hank and Supergirl are stunned by the pod that came crashing to Earth.

As Alex searches for Jeremiah and Kara investigates a series of alien kidnappings, the sisters realize they must break the rules to foil a new Cadmus plot. In an effort to get Snapper Carr to run a story that would help her stop Cadmus, Kara agrees to set up an interview between Snapper and Supergirl.

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.

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.

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.

Kara and Lena head to Stryker's Island to search for clues on how to defeat Lex. At the prison, Kara tries to befriend a nosy prisoner, Steve, who may have insight into what Lex was doing, while Lena studies an array of puzzles Lex left behind in the hopes of discovering his whereabouts. While there, a prison riot breaks out and Kara must find a way to stop the prisoners without revealing her secret identity as Supergirl. Meanwhile, Kelly encourages James to talk to someone for his PTSD, and Alex faces off against Ben Lockwood after he storms the DEO and acquires some powerful weapons to hunt Supergirl.

As Brainiac lays close to death after trying to stop Lex, Supergirl and team soar in to save him, engaging in an epic battle with Gamemnae. After beating Leviathan, Supergirl turns her attention to Lex who has used the Obsidian platform to brainwash half the world to love him and follow him at all costs, no matter what horrible things he does. Knowing how dangerous this makes her brother, Lena enlists the entire team – Alex, J’onn , Dreamer, Kelly and Brainiac – to help, but Supergirl realizes that the only way to truly stop Lex is to sacrifice herself.

Kara gains a new ally when the lightning-fast superhero The Flash suddenly appears from an alternate universe and helps Kara battle Siobhan, aka Silver Banshee, and Livewire in exchange for her help in finding a way to return him home.

An attack is made on the President as hot-button, alien vs. human rights issues heat up in National City. Supergirl and the DEO are assigned to protect the President and bring in the responsible party. Alex teams up with Detective Maggie Sawyer to investigate the case while Kara searches for a recently escaped Mon-El, who she fears may be the alien behind the assault.

When Jeremiah Danvers is rescued from Cadmus, Alex and Kara are thrilled to have their father back. The Danvers arrange a family dinner to celebrate but things go awry when a suspicious Mon-El starts to question Jeremiah about his sudden return.

Psi, a thief with psychic powers, attacks National City. Able to immobilize people by tapping into their worst fears, Psi proves a formidable opponent for Supergirl. Meanwhile, James and Lena are at an impasse, and Samantha starts her new job at L-Corp.

When Winn and the team discover an alien ship has crash landed deep underwater beneath National City, Kara/Supergirl is called in to investigate. Meanwhile, Hank attempts to bond with his father, Myr'nn. Samantha, eager for some understanding of the changes she has been experiencing, looks to her estranged mother, Patricia Arias, for answers.

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.

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 and Lena head to Kaznia to hunt down Lex. However, Eve Tessmacher sets a trap for the duo that could ultimately reveal that Kara is Supergirl. Meanwhile, Alex gets a long-awaited phone call, James practices using his powers, and Ben Lockwood takes a dangerous stand inside the DEO.

Kara turns on her friends and the citizens of National City after being exposed to Red Kryptonite makes her malicious and dangerous. Also, Cat appears on THE TALK to discuss Supergirl.

Armek, a White Martian, descends on National City intent on taking M’gann back to Mars to face her punishment as a traitor. Hank and Supergirl determine the best way to keep M’gann safe is to bring her to the DEO. However, when it is revealed that Armek shape-shifted into M’gann and is now loose in the building, the team realize he could be disguised as any one of them.