- "Can you believe it, sis? Three Luthors in the Oval Office. Only in America."
- —Lex Luthor to Lena Luthor
"The Quest for Peace" is the twenty-second and final episode of the fourth season of , and the eighty-seventh episode overall. It aired on May 19, 2019.
|Lex Luthor descends upon Washington, D.C. 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.|
- Melissa Benoist as Kara Danvers/Supergirl and Red Daughter
- Mehcad Brooks as James Olsen
- Chyler Leigh as Dr./Director Alex Danvers
- Katie McGrath as Lena Luthor
- Jesse Rath as Querl Dox/Brainiac 5
- Sam Witwer as Professor Ben Lockwood/Agent Liberty
- Nicole Maines as Nia Nal/Dreamer
- April Parker Jones as Colonel Lauren Haley
- David Harewood as J'onn J'onzz
Special guest star
- Bruce Boxleitner as President Phil Baker
- Brenda Strong as Dr. Lillian Luthor
- Azie Tesfai as Dr. Kelly Olsen
- Andrea Brooks as Eve Teschmacher
- Graham Verchere as George Lockwood
- LaMonica Garrett as Mar Novu/The Monitor
- Francoise Robertson as Sarah Walker
- Patti Allan as Margot Morrison
- Phil LaMarr as Malefic J'onzz
- Fraser Aitcheson as Lead Guard
- Steel Bey as Charlie
- George Boutros as Alien
- Keith Dallas as Al
- Darcy Hinds as Control Room Guard
- Raphael Kepinski as Kaznian General
- Kapila Rego as Lena's Assistant
24 hours before Kara was defeated by Red Daughter, the latter went back to Lex Luthor, saying 'sorry' for neglecting to kill Supergirl. He tells her to follow Eliza. Six hours before, Lex and Eve are then placing aliens into a battery-like chambers and uses their powers to control his Lexosuit. One hour before Lex and the President are in the Oval Office with the president scrutinizing Lex's sureness and the Kaznian assault starts. But just as Lex planned, he single-handedly stops the Kaznian invasion. Just as Red Daughter defeated Supergirl, she hears explosions, and meets Lex on the beach. She tells Supergirl is dead, but is shocked that Kaznia's attack happened early, as she was supposed to lead it. Lex reveals he did it, and without qualms, shoots down a Kaznian fighter, and seemingly kills Red Daughter
Kara tells Eliza and Alex that she needs to show the world the truth about Lex. In an AmerTek energy facility, Lex has Red Daughter put in one of the battery units where he reveals to her how stupid she was to have ever trusted him, as she could've found out the truth about him easily. He openly tells her that he hates Kryptonians. Lex seals Red Daughter into the pod, and she is feeling both betrayed and regretful. The president at that point names Lex as the new Director of Alien Affairs while an attempt is made on Ben Lockwood's life. However, he re-powers himself with the Harun-El serum and endures. The attackers were Eve-clones.
Somewhere else, Nia and J'onn have gone through the entryway and wound up on Shelley Island - where Lex has a greater amount of the batteries. At Lena's office, a still aloof Brainy uncovers what's new with Nia and J'onn. Kara and Alex arrive, and Brainy tells Kara he has already reacquired the evidence Secret Service erased. Kara then begins writing an exposé about Lex and his scheme and everyone involved in it. Lena receives an invitation from Lex to the White House.
Lena wasn't the only one to have received the invitation; Lillian did as well. Lex uncovers his arrangement to Lena and his mom: he's outfitting the forces of the aliens to utilize them as rechargable dispensable batteries to control the world. He likewise plans to utilize the main bunch of batteries to power up a new Claymore to assault and crush Argo City and hence, Superman. Eve pulls Lex away, and criticizes Lex for wasting energy on Superman. Lex disregards her, as his burning hatred towards his arch-nemesis can't be quenched. In Shelley Island, Al approaches J'onn and Nia and is ready to provide a distraction, so that J'onn and Nia can power down the island's power dampening pylons.
Kara finishes the article, and Alex is proud of it, even claiming it could win a Pulitzer. The article is transmitted to everyone in the country, and the truth about Lex's scheme becomes public knowledge. J'onn , Nia and Al start up a fight, allowing Nia and J'onn to disable the pylons. Nia then astral projects to Lena's office, but since nobody can see or hear her, Brainy has her write the answer on water vapor on the window. Kara and Alex go get James and head to the island along with Brainy.
As the heroes land on the island, Brainy learns the place has been transformed into an energy plant, where the power extracted from aliens is being used to power up a new Claymore, that is targeting Argo City. Suddenly, they are ambushed by Ben Lockwood and his Children of Liberty, who desire revenge against Lex for being his pawns and also kill every alien they can find, and fight starts. However, the Children are no match against Superfriends, even with Ben's Harun-El powers. Inside the facility, J'onn and Nia find the core, but they can't shut it down. Brainy arrives and claims the Claymore will fire soon. J'onn, however, intends to use his psychic powers to overload the core, claiming he won't let what happened to Mars happen again, despite Brainy's warning he will likely die. Nia chooses to help him by using her powers as well. As they charge up the core, Brainy's emotions start to return, and he encourages the two to go for it and admits he loves Nia. They succeed, and the core is powered down.
In the White House, Lex has outsmarted Lillian's poisoning attempt and the Claymore is almost ready to fire. But when the cannon doesn't fire, he angrily calls Eve, and she tells him there's a situation on Shelley Island. He immediately powers up his Lexosuit and flies there.
Kara, Alex and James are still fighting Lockwood, when suddenly, Lex arrives. James tells Kara to go, and she fights Lex. They crash inside, and Lex is shocked to see Kara alive. Lex is armed with Kryptonite, but Kara came prepared and activates Lena's armor, that can shield her from Kryptonite. Elsewhere, J'onn, Nia and Brainy free the aliens, and James and Ben manage to extract the Harun-El from each other, and Alex beats Lockwood to the ground. However, James lost an eye. J'onn, Nia and Brainy lead the aliens out. Among them is Red Daughter, who overhears Kara fighting Lex, and flies away. The power on Kara's armor starts to wear out, and Lex shoots her with a lethal blast of Kryptonite, only for Red Daughter to fly in between and take the blast instead. Both Kryptonians are blasted through wall, and Kara crawls to dying Red Daughter, saddened. Red Daughter, having grown fond of her template, says Kara was right about Lex, and encourages her to protect her people, as she protected hers. As Red Daughter dies, she merges with Kara, giving Kara a supercharge that allows her to defeat Lex easily, avenging Red Daughter as well. As Lex tries to escape by flying, his Lexosuit breaks. Kara flies, and begs Lex to let her save him, but due to his pride, Lex would rather die than be saved by a Kryptonian, and falls to the ground, exploding. The rescued aliens cheer at Supergirl, for saving the day.
However, Lex didn't die in the explosion. He ended up in his secret lab, where Lena ambushes him, having figured it out. She extracts the Harun-El from him, and pulls out a gun. Lex claims she isn't ruthless enough to kill him, no matter how much she despises him. But Lena surprises him by shooting him. He is surprised, but before he dies, he shows Lena a video, which proves to Lena that her best friend Kara is Supergirl, and how all her friends have lied to her, even Lillian. Lex dies, but he is happy, as he dies knowing he was not a fool.
At CatCo, the news show that after Kara's article was published, President Baker was removed from office by the 25th amendment, due to him working with Lex, and that the new president has re-enacted the Alien Amnesty Act. Red Daughter's identity was also revealed, clearing Supergirl's name, and Colonel Haley has been appointed as the acting Secretary of Alien Affairs, and openly thanks Supergirl for her service. Kara and Nia admit that for a while, it seemed that truth won't prevail, but they didn't give up, and the battle was won.
Later, Alex is walking with Kelly, and she claims that being close to Alex has made her feel whole, and Alex kisses her (fulfilling, what Sara Lance told her). They are passed by by Nia and Brainy, who have begun dating. The next evening all of the Superfriends have gathered for a game night at J'onn's place, finally able to relax after the ordeals. Lena arrives, and it seems she is okay, despite learning the truth about Kara. Kara and Alex discuss about Lena, and Kara intends to tell her she is Supergirl, but Alex advises her to let Lena enjoy the night, after so much pain.
The next day, George Lockwood is holding a press conference, where he intends to be a voice between the humans and aliens peaceful co-existence, with Nia being one of the reporters. He says that he has seen first hand, what hatred towards aliens has done to his family. Ben Lockwood has been incarcerated, and as he watches the news report, he is visibly shocked, how his son has totally rejected him. From the distance, Supergirl looks at George and smiles.
Lena is drinking in her office, and she smashes her picture of herself, Kara and Alex with the glass, showing she has succumbed to hatred for Kara having lied to her for years. Elsewhere, Eve is trying to hide in a disguise, but she is met by an old woman, who tells her, that they will always be watching her. Eve claims she did everything they wanted, but the old woman says that's not how her organization, Leviathan, works, and says they are everywhere.
Elsewhere, a portal opens, and a hooded figure emerges. He meets with Mar Novu, who says he was the one, who brought him here, so that he can back get at his brother, who betrayed him. The figure takes off his hood: he is revealed to be a Green Martian, and says he is coming for J'onn J'onzz. Finally, Mar Novu goes to Lex's lab, and does something to Lex's corpse.
- The title is a nod to the 1987 film Superman IV: The Quest for Peace.
- As they had done in the previous episode, the Supergirl logo is done in Russian.
- This marks the last appearance of Sam Witwer and April Parker Jones as series regulars as they did not return in Season 5. Although Witwer did return as a guest star in the 100th episode "It's a Super Life".
- When Lex explains his plan to attack Argo City, he calls Superman "the Man of Yesterday." In "", Manchester Black calls Supergirl "the Girl of Yesterday."
- When Brainy's emotions begin to come back, he repeats the phrase "little boxes" to himself, referencing the advice Lena gave him in "".
- In the penultimate scene, it's revealed that Eve has been working for an organization called Leviathan. In the comics, Leviathan is an offshoot of the League of Assassins led by Talia al Ghul; the League and Talia's father, Ra's al Ghul, appear in the third season of , while Talia recurs in the fifth season and appears once in the seventh and eighth. Both Ra's and Talia once appeared in the same episode ("" in the first season of ,
- This episode marks the debut of J'onn J'onzz's brother, Malefic J'onzz.
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