Warning: The following contains SPOILERS for the 100th episode of Supergirl.
The 100th episode of Supergirl took Kara Zor-El on a walk down Memory Lane and into a multiverse of possibilities as she was offered a chance to rewrite her past. This journey came from an unexpectedly helpful source; a reformed Mr. Mxyzptlk.
Supergirl first encountered the reality-altering Mr. Mxyzptlk in the season 2 episode "Mr. & Mrs. Mxyzptlk." The mischievous imp from the Fifth Dimension claimed to had fallen head-over-heels in love with the Girl of Steel and was determined to win her over, no matter how much she insisted she wasn't interested. The 100th episode of Supergirl sees Mr. Mxyzptlk reenter Kara's life, but with a noble purpose in mind this time.
Placed on trial for abusing his powers after his last encounter with Supergirl, Mr. Mxyzptlk was sentenced to return to every person he had ever wronged and use his powers to help them. In Kara's case, he suggests that he can alter Kara's past and allow her to tell her former best friend Lena Luthor the truth about her secret identity before Lex Luthor did so in Supergirl's season four finale. The following trip through time was hilarious and horrifying in equal measure, as Kara and Mr. Mxyzptlk traveled through a variety of timelines where life for Kara and Lena became increasingly worse the more she tried to change things.
The One Where Supergirl Dies
Based On: Supergirl season 4, episode 2, "Fallout."
What Changed: In this reality, Kara revealed her secret identity to Lena shortly after the resignation of President Olivia Marsdin (who was revealed to secretly be an alien) just before they were attacked by Mercy Graves. When Kara returned to Lena's office after repelling the attack, Lena was gone.
Heartbroken that she'd been lied to, Lena flew to Metropolis and sought the advice of Samantha Arias, the woman who had been the Kryptonian World-Killer Reign until Lena synthesized a Kryptonite-based formula that freed Samantha from Reign's control. Remembering how cruel Supergirl had been towards her when she discovered she was making synthetic Kryponite but how Kara acted exactly the same towards her, Lena was left wondering which was the real Kara. Samantha assured Lena that she was a hero and that she believed Lena was good, no matter what Kara thought.
How It Ended: Lena was in Metropolis at the end of "Fallout," when Mercy Graves and the Children of Liberty seeded Earth's atmosphere with Kryptonite. Supergirl died since Lena wasn't in National City, where she could immediately call upon the resources of the DEO and LuthorCorp to save Supergirl's life, as she did in the original timeline. This scene ended with Alex Danvers visiting Kara's grave and tearfully apologizing for not visiting her on Thanksgiving, breaking their usual tradition of spending the holiday together.
The One Where Lena, Mon-El and Reign Die
Based On: Supergirl season 3, episode 17, "Trinity" and episode 18, "Shelter From the Storm."
What Changed: In this reality, Supergirl revealed that she was Kara Danvers to Lena, just after Lena confessed that she had known Samantha Arias was Reign for three weeks and that she was creating synthetic Kryptonite as part of a plan to help her. Having already spent the better part of a month agonizing over one of her friends being a secret Kryptonian biological weapon, the revelation that Kara had been keeping a secret from her instantly embittered Lena against her. This resulted in Lena not joining forces with the DEO in fighting Reign and the rest of the World-Killers on the day they caused a solar eclipse. It also had the side-effect of keeping Mon-El in the past for longer, as they didn't defeat the World-Killer Pestilence on the day of the eclipse.
How It Ended: Supergirl and Mon-El confronted Reign together at the farm belonging to Samantha's mother, Patricia Arias. The battle went poorly, with Reign on the verge of killing Patricia when Lena showed up with an electrical weapon that seemingly shocked Reign into submission and allowed Samantha to take control of her body again. For a moment it seems that everything would end happily, with Lena having found a way to save Samantha without using the Kryptonite that had caused Kara to stop trusting her and the rift between the two friends seemingly healed.
Unfortunately, it turned out that Reign was not as dormant as Lena had thought and Reign took over Samantha's body once again, shooting Lena in the back with her heat vision. An enraged Mon-El grabbed Lena's device and charged Reign, discharging the weapon into her at close-range as Reign fired a heat-vision blast through his chest. In the end the world was safe, but Supergirl lost three friends in the process.
The One Where Kara And Lena Become Partners
Based On: Supergirl season 2, episode 12, "Luthors;" season 3, episode 4, "The Faithful;" season 4, episode 3, "Man of Steel;" and season 5, episode 6, "Confidence Women."
What Changed: In this reality, Kara told Lena that she was Supergirl shortly after the two women first met, in an alternate take on a flashback scene from Supergirl season 5, episode 6, "Confidence Women." Still hurting from her betrayal at the hands of her old school friend Andrea Rojas, Lena was stunned by the confidence Kara placed in her, despite barely knowing her and Kara being the first person who didn't look at her as a supervillain in training because of the rest of her infamous family. This led to the two forming a partnership to make National City a better place together.
The events of the Supergirl series played out in a wildly different fashion thanks to this choice. Lena testified against her adoptive mother, Lillian Luthor, as she did in the season 2 episode "Luthors." This time, however, the public came to trust Lena, despite her refusal to reveal Supergirl's secret identity under oath. In the original timeline most assumed that Lena was in league with her mother's work with the anti-alien group Cadmus, but publicly partnering with Supergirl gave her a credibility she'd never enjoyed as a Luthor. Another major change was that the Cult of Rao, which was originally formed to worship Supergirl, became a larger but more benign organization that neither Kara nor Lena took all that seriously.
How It Ended: In this reality, steel-mill owner Peter Lockwood fell in with the Cult of Rao and turned his mill into a church. He, his grandson and his daughter-in-law reportedly died after they jumped off the roof of the mill expecting Supergirl to save them. This left Peter's son Ben Lockwood embittered against Supergirl, whom he felt wouldn't be worshiped if people knew she lived a normal life like everyone else when she wasn't saving the world.
Adopting the Agent Liberty identity as in the original timeline, Lockwood and his associate Otis Graves abducted both Lena Luthor and Thomas Coville, the founder of the Cult of Rao. Agent Liberty threatened to kill both Lena and Coville if Supergirl did not reveal her secret identity to the world. Kara complied with his request and, true to his word, Agent Liberty revealed where the two hostages were being kept. Unfortunately, this did not stop the Children of Liberty from later killing everyone who had ever been associated with Kara Danvers, from her co-workers at the DEO to her former boss Cat Grant.
The One Where Kara and Lena Never Met
Based On: Supergirl season 2, episode 1, "The Adventures Of Supergirl."
What Changed: In this reality, Supergirl and Lena Luthor never met, so Kara never had a chance to ruin their friendship by hiding her secret identity. Unfortunately, this also meant that Supergirl didn't get involved when Lena Luthor's helicopter was attacked by drones; an attack that Kara stopped in the Supergirl season 2 premiere.
How It Ended: Severely injured in the helicopter crash, Lena was redirected from a hospital to Cadmus, where Lillian Luthor used her as a test-subject for the process that created Metallo. Tortured by her family and without a friend in the world, Lena became as cold and hard as the metal implanted into her body. Two years later, she escaped and killed Lillian and Lex Luthor, destroying half of National City in the process.
Lena ruled what was left of the city with an iron fist, turning Reign and Brainiac-5 into her personal enforcers. Lena also reprogrammed all of her family's Lexosuits into robots under the control of her Hope AI, giving them strict orders to enforce her ideals of Non Nocere upon pain of death. This left all of National City's heroes in hiding, desperate for a way to fight back against Lena's superior technology, which was empowered by her draining the energy of the Fifth Dimension. In the end, there was no way to defeat her and restore the Arrowverse to what it had been apart from Supergirl helping Mr. Mxyzptlk recover his magical hat and enough power to put things back the way he had found them.