SPOILER WARNING: This article contains major spoilers for "Tremors," the latest episode of Supergirl.
Lena's big moment has finally arrived on Supergirl. As promised in the episode synopsis, "Tremors" saw the youngest Luthor reveal her true feelings to Kara. This led Lena to betray Kara, leaving her trapped in the Fortress of Solitude for the time being.
At the start of the episode, Lena made a major step toward Project Non Nocere, her plan "to rid humanity of its instincts to hurt each other." She convinced Supergirl to take her up to the Fortress of Solitude to protect her from Rama Khan, the leader of Leviathan. There, she stole Myriad, a device capable of large-scale disbursement. She hopes to use it to spread Project Non Nocere across the world in an effort to achieve world peace.
Lena acquired Myriad as Supergirl fought Rama Khan. However, Kara caught her just as she was leaving. When Kara demanded to know what Lena was doing, she simply replied, "I have Myriad because I'm using you, like you used me."
"I never used you," Kara protested.
"Do you remember when you finally told me you were Supergirl?" Lena asked. "You were weeping, big crocodile tears. Well, I wept real tears, bitter tears, over you weeks before."
Kara was horrified. "How long had you known?"
"I found out the day I killed my brother," Lena replied. "Lex used his transmatter portal watch. I knew exactly where he'd go: to the cabin we loved when we were children. So I was there waiting for him. I had the gun ready. Loaded. I could feel the weight of it in my hands. Every fiber in my being rebelled -- I didn't want to do it -- but I knew I had to, because if Lex lived, the world wouldn't be safe. My friends wouldn't be safe."
"So I forced myself to pull the trigger," she continued. "I shot my own brother in the chest. His final words to me were that I was a fool, that my best friend -- that every friend I had -- was lying to me. With his dying breath, he told me that you were Supergirl."
"Lena, you have to believe me, I never meant to humiliate you," Kara insisted.
"When I came to this city, I promised myself that I would never trust anyone again, and then I met you," Lena recalled. "You chipped away at my armor with your warmth and your earnestness and you convinced me to trust in people and friendship again and, against my better judgement, I did -- all the while telling you about my Achilles heel: betrayal."
"I confided in you that everyone in my past had betrayed me, about how much it hurt to have someone you love lie to you and betray you," she said, sobbing. "I spelled it out to you over and over again, essentially begging you not to violate my trust, begging you not to prove that once again I was a fool. You reassured me, ad nauseum, that you would never lie to me, that you'd never hurt me. All the while, there wasn't a single honest moment in our friendship."
"No!" Kara cried. "That is not true!"
"I killed my brother for you!" Lena yelled. "For our friends! Don't you understand what you've done?!"
The outburst stunned Kara into silence, but eventually she carried on, saying, "When you found out, why didn't you come to me right away, so I could explain? If you hate me so much, why not out me at the Pulitzer party? Why pretend to still be my friend this long?"
"Because I wanted you to experience what you did to me," Lena replied quietly. "Feel what I felt."
"I understand your pain and your fury, and you're right," Kara insisted. "I made a big mistake, maybe the biggest mistake of my life, but please -- Lena, please don't leave the Fortress with Myriad. Please don't let my mistake push you to do something terrible. You are a good person!"
"No, you don't ever get to tell me who or what I am again," Lena spat. At that, she pressed a button on a secret trigger she was holding. This enabled the Fortress' defense system to trap Kara. Lena had rewired the Fortress' supercomputer while Kara's back was turned.
When Kara asked if she was trying to kill her, Lena said, "No, Supergirl. I'm not going to kill you. I'm not a villain. You shouldn't have treated me like one." With that, she activated her transmatter portal watch and left Kara behind, trapped in the Fortress' security system.
Airing Sundays at 9 p.m. ET/PT on The CW, Supergirl stars Melissa Benoist as Kara Danvers, David Harewood as Martian Manhunter, Mehcad Brooks as Jimmy Olsen and Chyler Leigh as Alex Danvers.