Mxyzptlk returns with a proposition for Kara; Kara must decide if she wants to change history to become friends with Lena again.
- Mxyzptlk says who he is, and starts to announce Kara's secret identity to the world. Kara quickly gets him into her apartment and asks what he wants. He says that he took on his earlier form and accent to impress Kara, and she tricked him into going back to the fifth dimension. Mxyzptlk explains that now he can only use his powers to help those he's wronged, Kara is the last such person, and when Kara wonders what she needs him to help with, Mxyzptlk plays her confrontation with Lena on the TV. He tells Kara that is' her fault that Lena is now her worst enemy because she hid the truth from Lena so she found it out from Lex.
The imp offers Kara the chance a do-over: sending her back in time to tell Lena that she's Supergirl before she finds out from Lex and find out what the world would be like if she controlled the narrative. Mxyzptlk admits that he understands why Kara doesn't trust him, and under fifth dimensional law gives her a recording of her saying his name backwards in case she ever needs to banish him back to his home dimension. An exasperated Kara immediately plays the recording and Mxyzptlk disappears.
The video of Lena keeps playing on the TV, and John comes in with pizza as the image disappears from the TV. Kara and Alex tell him what happened, and Kara contemplates taking Mxyzptlk up on his offer and changing history. Alex and John warn against it, but Alex tells Kara to do it if she thinks it's the best chance to stop Lena before she hurts someone.
Mxyzptlk emerges from the refrigerator where he's been eavesdropping, and Kara makes him promise to set everything back to normal if something goes sideways. Since it works betters alone, Mxyzptlk sends Alex and John away to their "happy place": a paint gun camp. Mxyzptlk explains that he'll show Kara relevant parts of her life and she'll tell him when to "pause", and he'll send her back so she can change history. Then Kara will come back to their present, they'll see the new timeline, and Mxyzptlk will make it reality.
Kara and Mxyzptlk settle down to watch the past, and Mxyzptlk realizes that Kara wants to find a moment when she made Lena feel like an idiot. He "plays" the scene where the LexCorp jet is going to crash and Lena flies it. Kara thinks that it's awful, and Mxyzptlk agrees. He sends Kara to a moment right before Mercy attacked, when she and Lena have takeout in Lena's office. Kara tells Lena that the world has made Lena cynical because she's been betrayed so many times, and admits that she's betrayed her as well. She reveals that she's Supergirl and didn't know how to tell Lena.
An alarm goes off and Kara speeds off to deal with it. When Supergirl comes back, she discovers that Lena is gone. Back in the present, Mxyzptlk shows Kara that Lena went to Metropolis and met with Sam. Lena tells Sam that Kara is Supergirl, and says that Kara was horrible. Now she figures that since Kara thought Sam was a villain, and she thought Lena was a villain, she might really be a villain. Sam reminds Lena that she made Kryptonite to split her from Reign and save her life, and was always the hero. Lena thanks Sam and hugs her, and the present Kara realizes that Lena might never forgive her.
Present Kara figures that she can live with it if Sam stops Lena from going down a dark path. Mxyzptlk says that Kara literally can't live with it, and in the past Agent Liberty pumps green Kryptonite into the atmosphere. In the new timeline, Lena doesn't save Supergirl. Alex visits Kara's grave and says that they--and she--are lost without her. Mxyzptlk stops the film and breaks into tears, offering Kara a Kleenex. Kara figures that she has to go back further, and Mxyzptlk sends a laptop video to the point where Supergirl told Lena not to make Kryptonite ever again.
Mxyzptlk continues rewinding to the moment when Supergirl asked Lena if she had any more Kryptonite. Kara goes back and reveals her secret identity to Lena. Lena is angry that Supergirl didn't help her when she tried to split Sam from Reign, and Supergirl says that she thought she was protecting Lena. Supergirl wants to move past it so they can work as partners, but Lena says that she'll never be partners with Supergirl after she's proven she can't be trusted. The Worldkillers eclipse the sun as in the original timeline, and Supergirl weakens...
...and Mxyzptlk returns Kara to the present. Mxyzptlk says that they're not going to rely on Kara's memory, and says that Lena won't help the DEO against the Worldkillers. He shows the altered past where Mon-El doesn't go back to the future, and Kara sends Supergirl to the past Mon-El to get his perspective. At the Arias house, Mon-El tells Patricia to hide and she refuses to abandon Sam, but goes to a safe room. Supergirl tells Mon-El that she told Lena about her secret identity and wonders what she should do since Lena didn't take it well. Mon-El says that he and Lena have a lot in common, because they were both betrayed by their family, but says that Supergirl lied to protect Lena because Lena is Supergirl's friend. He assures Supergirl that once Lena understands, she'll forgive Supergirl. Supergirl worries that she won't be able to forgive herself, and Mon-El points out that Supergirl has always forgiven him no matter how many mistakes he's made... and Supergirl deserves the same compassion she shows others.
Reign arrives, and Mon-El and Supergirl fight her. Patricia comes out and tries to get through to Reign, and Lena arrives and uses an energy blaster on Reign. She begs Sam to come back to them, and Reign vows to kill them all. Lena blasts her again, killing Reign, and Sam takes over and looks at Lena, shocked. Patricia begs Sam's forgiveness and hugs her, and Lena tells Supergirl that she saved Sam without Kryptonite. Reign heat-visons Lena, killing her. Mon-El attack Reign and Reign kills him as well with her heat vision. The backlash kills her as well as Mon-El uses Lena's blaster, and a sobbing Patricia begs Sam to come back to her.
Kara has Mxyzptlk return her to the present, and says that they have to go back to the very beginning. Mxyzptlk has lost his Wi-Fi connection but has Betamyx, and he plays a video tape of when Lena first moved to National City. Kara says this time she's going to do it right, and Mxyzptlk sends her to the moment. As Lena says that she didn't come to National City to make friends, Kara tells her that she's Supergirl, and Lena is thrilled. She wonders why Kara is trusting her with her secret, and Kara explains that she needs Lena to know without her doubt that Lena can trust her. They shake hands af6er agreeing to change the world.
Soon, Supergirl and Lena clean up National City, and testify at Lillian's trial. The defense attorney questions Lena about Supergirl's secret identity, insisting that Lillian has the right to know the name of the "officer" who arrested her. Lena pleads the Fifth as Supergirl watches from the back of the courtroom, and Thomas Coville tells a reporter that nothing as trifling as the laws of man can stand in the way of Supergirl and Lena's divine victories. He declares that the greatest power is that of a Kryptonian teamed with a Luthor, and the crowd of his followers cheer him.
Watching, Kara tells Mxyzptlk that Lena gained credibility by working with Supergirl. However, they watch as Ben approaches Lena and explains that his father Pete converted his factory to a Church of Rao and then died along with Ben's family. They believed that Supergirl would save them and they didn't, and he asks Lena who Supergirl is. Ben says that Supergirl hides in plain sight as a human, and if people knew who Supergirl was then they would stop worshiping her. He says that Lena is going to tell him, and when Lena refuses, Ben reveals that Lena's bodyguard is a holographically disguised Otis, and they've already abducted Coville. They abduct Lena as well and leave.
Later, Ben broadcasts to National City that Supergirl lives a secret human life. He announces that Supergirl will reveal her identity, or he'll asphyxiate Lena and Coville. Winn and the others at the DEO can't track Ben's signal, and Supergirl worries that her fear of someone knowing her identity has threatened that someone's life. She worries that if she reveals her identity to save Lena, she puts all of her other loved ones at risk. Winn tells her not to worry about the rest of them because Lena needs Supergirl's help, and Supergirl taught them to protect themselves..
Later, Kara reveals her identity on TV. Ben watches along with Lena and Coville as they slowly asphyxiate, and Ben tells Otis that they'll keep their word and tell Supergirl the hostages' location. Supergirl flies there and frees the hostages, and Lena hugs Supergirl in relief. In the present, Kara wonders if it's the timeline she wants. Mxyzptlk skips ahead to the ending, where Agent Liberty and his followers target each of Supergirl's friends and kills them. Kara figures that telling the Lena the truth always makes things worse, and admits that she thinks Lena would be better off if they were never friends.
Mxyzptlk shows Kara a timeline where Lena never found out she was Supergirl. National City is devastated and Kara's apartment is destroyed. His powers don't work and he can't undo the timeline. LexCorp soldiers and a "Hopebot" confront them and announce they are out past curfew. The Hopebot is immune to Kara's powers, but Dreamer destroys it from behind.
Dreamer takes Supergirl and Mxyzptlk to the headquarters of the resistance, including Alex, Winn, Mon-El, John, and Kelly wearing the Guardian armor. Mon-El recognizes Mxyzptlk, and Supergirl assures him that Mxyzptlk is reformed. Mxyzptlk says that he's going to try to be better and shakes hands with Mon-El. Supergirl asks what's going on, and Lena appears on a city-wide TV broadcast and announces that Supergirl is a terrorist and at large, working with Mxyzptlk capable of mass destruction. Lena has deployed Hopebots throughout National City to "protect" the citizens, and asks every citizen to report suspicious activity. She insists that her goal is to protect everyone from harm.
The Resistance explains that when Lena first came to National City, Lex tried to have her assassinated but failed. Lena was seriously injured and disappeared for two years, then came back different. The loneliness and trauma drove her mad. She killed Lex and now she rules what's left of the city with Reign and Brainiac-5 as her enforcers. Lena captured Brainiac-5 and stripped him of his emotions. Supergirl realizes that she made a terrible mistake, and Mxyzptlk warns that they need his powers. Mon-El figures that Mxyzptlk has lost his powers because Lena tapped into the fifth dimensional to acquire a power source for her Hopebots. Winn remembers having met Hat, and Mxyzptlk admits that the Hat's hat contains fifth dimensional magic and he lost it to Hat in a poker game. If they get Mxyzptlk the hat, he can use his powers again.
Reign and Brainiac-5 burst in through the windows and Reign tells the Resistance to surrender or die. Dreamer blocks her heat vision blast, and Winn and Mon-El take Mxyzptlk to get the hat while the others hold Lena's attackers off. Supergirl parries Reign's heat vision with her own, and John flies her through the ceiling. As Supergirl flies after them, Dreamer tries to get through to Brainiac-5, removing her mask to show him her face. Brainiac-5 says that he'll enjoy defeating them, and Kelly and Alex prepare to fight with Dreamer.
In the skies above National City, Reign attacks John and Supergirl. John tells Supergirl to go after Lena while he handles Reign.
Brainiac-5 holds off all three heroines.
Supergirl flies into Lena's office and calls to her. When she says that she's Kara, Lena reveals herself and wonders how Kara is. Supergirl says that she's just a friend and Lena doesn't know her, but they want the same thing: to do no harm. Lena asks where she was when Lex blew her out of the sky, and Lillian experimented on her instead of fixing her. She then opens a panel on her chest, revealing her green Kryptonite heart. Supergirl weakens from the radiation, and Lena blasts her with a Kryptonite beam.
Brainiac-5 knocks out Dreamer and parries Alex and Kelly's blows.
Mxyzptlk, Mon-El, and Winn enter the lab where the hat is stored.
At the DEO, Reign blasts John down with her heat vision. Mon-El realizes that John is under attack, and Winn finds something that John needs. Mon-El comes out and tosses John the Staff, and Reign flies at him.
Dreamer recovers and immobilizes Brainiac-5.
John stabs Reign in the chest with the Staff and she disintegrates.
Lena blasts Supergirl through an inside window, kneels over her, and says that Supergirl doesn't know the first thing about fate but she does. Supergirl refuses to fight Lena, and Lena says that then she'll die screaming and unleashes more Kryptonite radiation.
Winn finds the hat and tosses it to Mxyzptlk, who uses it to restore his powers and return himself and Kara to the present. Mxyzptlk says that they need to find a time when Kara can tell Lena the truth. Kara refuses, saying that she's realized that there will always be consequences no matter when she tells Lena the truth. If she can't eliminate her mistakes, she'll have to live with them. Kara tell Mxyzptlk that he still helped her by freeing her from her burden of guilt, and gives him the recorder and sets him free. Mxyzptlk says that when he was with her, saving the world, it felt like he was liked for being himself. He tells Kara that magic has to be found, not forced. Kara thanks him for everything and he heads off to Mardi Gras in the 1780s.
Kara grabs one of Mxyzptlk's Betamyx tapes labeled, "The One When Lena Decided to Work With Lex", then flies to Lena's apartment. Lena figures that Supergirl is there to tell her not to work for Lex, or ask for forgiveness. Supergirl says that she's recognized she made a mistake but she can't change the past. Whether Lena forgives her is her choice, just like it's her choice whether she works with Lex. Supergirl says that she's done blaming herself for Lena's bad decisions. Lena wonders why Supergirl is there, and Supergirl tells her that Lena is accountable for her own actions. She'll be there for Lena if she forgives her, and if she works with Lex or goes through with whatever they're planning, she'll stop them... like she would with any other villain.
Back at her apartment, Kara meets with Alex and John, and discovers that Mxyzptlk has left her a lot of food. As they eat, Kara says that seeing the other timelines made her realize that second-guessing herself accomplished nothing. Kara thanks them for being there for her with her best and worst moments, and it means the world to her.