- "People don't remember this, Kara, but Clark and Lex Luthor used to be best friends. For years. And Clark believed in Lex for the longest time. No matter what people said. No matter what kind of proof he saw with his own eyes. No. Lex was his friend. But Clark was wrong."
- "Lena is not Lex."
- —James Olsen and Kara Danvers
|After Metallo breaks out of prison and frees Lillian Luthor, the police blame Lena for his escape and arrest her. Despite overwhelming evidence, Kara refuses to believe her friend is guilty and fights to clear Lena's name. Flashbacks reveal how Lena came to be a Luthor. Meanwhile, Alex introduces Maggie to the gang.|
- Melissa Benoist as Kara Danvers/Supergirl
- Mehcad Brooks as James Olsen/Guardian
- Chyler Leigh as Dr. Alex Danvers
- Jeremy Jordan as Winn Schott
- Floriana Lima as Detective Maggie Sawyer
- Chris Wood as Mon-El
- David Harewood as Director J'onn J'onzz/Martian Manhunter and Hank Henshaw/Cyborg Superman
- Brenda Strong as Dr. Lillian Luthor
- Katie McGrath as Lena Luthor
- Frederick Schmidt as John Corben/Metallo
- Peter Gadiot as Mr. Mxyzptlk
- Andrea Brooks as Eve Teschmacher
- Ian Gomez as Snapper Carr
- Marshall Bingham as Guard #2
- Ian Butcher as Lionel Luthor
- Dawn Chubai as Newscaster
- Aidan Fink as Young Lex Luthor
- Robert Zen Humpage as Fish Head
- Damian Mavis as Prison Guard
- Camille Marty as Young Lena Luthor
- Judith Maxie as Judge Hobart
- Walcott Morgan as Creepy Guard
- Angelo Renai as Lillian's Attorney
- Desiree Zurowski as US Attorney Webb
In a flashback to twenty years ago, Lillian Luthor is teaching her son Lex chess. Lionel arrives home with newly adopted Lena, which Lillian is apprehensive to at first. Lex offers to teach Lena chess which causes Lillian to warm up to the idea of Lex having a sister.
In the present, Kara and Mon-El are at Al's Dive Bar watching a newscast of Lillian's upcoming trial and awkwardly catch up with each other. Kara then meets up with James, Winn, and J'onn. Alex arrives and introduces Maggie to the group as her girlfriend. Mon-El brings over drinks and congratulates the couple. Maggie challenges Winn to a game of pool. Kara and James discuss the trial. Kara defends Lena's actions, but James is convinced that Lena is using Kara.
Later, Kara visits Lena to comfort her and bring her donuts. Kara advises her to go see her mother, despite the rift between them.
Lena visits her mother in prison. Lena tells Lillian that growing up, she felt unwanted and like an outsider to the family. Lillian tells Lena that Lionel had an affair with her biological mother. After she died, the Luthors adopted her but Lionel did not like Lillian spending time with Lena so she focused on Lex. Now, Lillian wished to have a second chance with Lena. Meanwhile, John Corben receives a package of kryptonite in prison.
At the trial, Corben is being cross-examined. He claims Cadmus saved him and that aliens should be wiped from Earth. He proclaims that Lillian wants to save them all but Supergirl silenced her. Corben then fires a kryptonite beam at the prosecutor and frees Lillian. Supergirl tries to stop them but Corben fires a beam, destroying a crane. Supergirl is forced to save the people from the falling crane as the two get away.
Later, Maggie is telling Kara and Alex that the prison only had one visitor last night, Lena Luthor. Kara goes to Lena and tells her of the situation, but Maggie and the NCPD show up and arrest Lena due to damning security footage showing her taking kryptonite out of her safe.
Kara walks into CatCo in chaos. James tells her they are crashing the cover and putting Lena on the cover. Kara defends her friend to James and Snapper, but James sides with Snapper, believing that Lena is deceiving Kara. Kara bumps into Eve, who tells her that she and Mon-El broke up because he would only talk about Kara during their dates.
Corben arrives and breaks Lena out of prison. Guardian tries to intervene but Corben overpowers him and escapes with Lena.
Alex tends to James's wounds at the D.E.O. infirmary. James informs them that something was wrong with Corben's kryptonite core. Kara promises them that she will save Lena. James and J'onn insist she is guilty. James talks about Clark and Lex's history and prior friendship. He asks Kara why she isn't willing to trust him, but Kara leaves.
Mon-El catches Kara venting in the training room. He tells Kara that he admires her ability to believe in the good in others that other people don't. Kara admits that she was mad that Mon-El was dating Eve, but when he asks her why she leaves without answering.
In the back of a Cadmus truck, Lena is angry that Lillian freed her. Lillian assured her that the public wouldn't have believed her anyway. She pleads for Lena to join Cadmus. Meanwhile, Corben's condition worsens. They arrive at Lex Luthor's mountain vault, with Lillian explaining that Lex kept caches stored with technology that could stop Superman. Lillian tells Lena about how any Luthor can open the vault. Lena then realizes that Lillian only freed her to open the vault. She refuses to open it for them, but Hank Henshaw forces hand onto the scanner, opening Lex's stash.
At the D.E.O., Winn discovers that Cadmus hacked the security footage to make it appear as if Lena took kryptonite from the safe to frame her. The real culprit was Henshaw. They scan for and find a kryptonite signature in the mountains. The scans also show that the kryptonite is synthetic and decaying rapidly. Kara races off to save Lena.
Cadmus plunders Lex's vault. Lillian discovers one of Lex's finished projects stored in a small cube. Supergirl arrives to stop them, but Lillian disables her with a sonic grenade. Corben's kryptonite heart starts fluctuating. He fires a kryptonite beam at Supergirl and holds her hostage. Martian Manhunter arrives on the scene and saves Supergirl. Supergirl grabs Lena and they escape just as Corben goes critical and explodes.
The next day, Kara arrives at CatCo and gloats to Snapper about being right about Lena. Snapper brushes it off and tells Kara to get them an exclusive with Lena. James apologizes to her for being wrong about Lena. Kara tells him that she wants them to be friends again. They both agree on a game night. At Lena's office, Lena thanks Kara for the favorable story and being there for her when no one else was. After Kara leaves, Lena reminisces to twenty years ago about beating Lex at a game of chess as she stares at her office chessboard in present day.
Mon-El arrives at Kara's apartment and explains that they are both really different but that despite his flaws, Mon-El proved her wrong about him. They nearly kiss, until a portal opens and Mxyzptlk interrupts them, professing his love for Kara.
- The episode's title is a reference to the Luthor family: Lex, Lena, Lionel, and Lillian.
- This episode confirms that Lena is physically three years younger than Kara.
- This episode introduces Lex as a young boy.
- This episode establishes that Lena and Lex's father was named Lionel. This name was first used in Smallville, in which Lionel Luthor was played by John Glover, and then introduced into the comic canon in the 2003 limited series Birthright.
- Among the items in Lex's mountain vault include his warsuit from the comics, which Lex used to even the playing field against Superman.
- In , the suits became known as Lexosuits, which would appear more in subsequent seasons.
- When Kara mentions Superman getting rid of the D.E.O.'s Kryptonite, she calls him "Clark" with Maggie behind her (albeit on the phone).
- J'onn declares that "Kryptonite is usually the most stable of elements". However, Kryptonite is radioactive, the opposite of stable.
- When fighting Metallo the first time, he shoots a continuous blast. When Supergirl is shown though, the blast goes by her in multiple shots.
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