|Supergirl and team take on Selena in an epic battle for Earth.|
- Melissa Benoist as Kara Danvers/Supergirl and Red Daughter
- Mehcad Brooks as James Olsen/Guardian
- Chyler Leigh as Dr. Alex Danvers
- Jeremy Jordan as Winn Schott
- Katie McGrath as Lena Luthor
- Odette Annable as Samantha Arias and Reign
- Chris Wood as Mon-El
- David Harewood as Director J'onn J'onzz/Martian Manhunter
- Carl Lumbly as M'yrnn J'onzz
- Betty Buckley as Patricia Arias
- Chad Lowe as Thomas Coville
- Erica Durance as Alura Zor-El
- Emma Tremblay as Ruby Arias
- Jesse Rath as Querl Dox/Brainiac 5
- Amy Jackson as Imra Ardeen/Saturn Girl
- Anjali Jay as Selena
- Winsome Brown as Ayala
- Rosemary Hochschild as Vita
- Barbara Patrick as Woman
As the terraforming of Earth goes on, National City and the Earth are hit by a number of different natural disasters. Supergirl, Mon-El, Guardian, Alex and Alura help out people in any way they can. Superman is also helping out elsewhere. They get support from Imra and Brainy, as Imra shields a large water spout, which Kara and Alura freeze, and Brainy destroys. Guoardian heads to a burning building, where a panicked mother is trying to head back to save her son. Guardian takes off his mask to calm her down and saves the boy. Meanwhile, J'onn and M'yrnn arrive at a fissure in Madagaskar, where M'yrnn intends to commune with Earth to stop Reign. He shows one last memory to J'onn: the story of Martian scrolls. J'onn is in tears, as M'yrnn will not survive this ordeal, but M'yrnn assures him he will never be gone, and tells his son to live as he has. He then uses the Staff of Kolar to phase into Earth's core. Reign is stopped as a result, and disaster is averted. The Children of Juru are baffled about, what just happened.
Meanwhile, Samantha is in Juru, attempting to reach the Fountains of Lillith, in order to gain the necessary power to destroy Reign for good. She encounters Patricia, but she refuses to listen to her, thinking it's a trick. She reaches the fountain, but Patricia stops her from drinking, as Samantha was just about to drink from the wrong fountain. Samantha refuses to listen. Patricia then asks Samantha to trust her, and drink from the real fountain. In the end, Samantha reluctantly drinks from it.
The Children of Juru try to open the hole in order to get Reign back, but the fail to notice Coville is still alive. He sees J'onn's spaceship, and quietly tries to reach it.
At the DEO, Mon-El and Kara talk until Imra arrives. Mon-El thanks Imra for coming, but she tells him that being away from him has made her realize their marriage was never what it should have been, and they annul their marriage, but remain as friends and teammates. Later, Mon-El is sitting in the conference room and Winn tries to talk with him, but Brainy comes to tell them both the future needs them both. Even though the Legion stopped the birth of Blight, a new threat in the form of Brainy's distant relative will come to terrorize the future. And Brainy's presence as a 12th level intellect would be too risky for him to be there. So he tells Winn to takes his place in the Legion, as his intellect, while small compared to Brainy, is exactly what is needed to fight this threat. Winn claims he's not up for something like that, as he made something that couldn't save anyone. But Brainy then shows him the drawing he made, and reveals that in the 31st century, it is regarded as an artifact, as it serves as the basis of many technological breakthroughs in the future technology. Because of it, that technology has saved millions of lives Winn finds that hard to believe.
James comes to see Winn, who is still unsure what he should do. James tells that he took off his mask, and found some clarity. Meanwhile, in the Sanctuary, Reign finally resurfaces and is willing to try again. However, none in the sanctuary saw Coville, who managed to activate the homing beacon on J'onn's ship just before dying. The signal from the beacon is picked up at the DEO, and Kara tells everybody this will be the last battle. Lena and Alex still watch over Samantha, who still hasn't woken up, or gained powers. In Juru, Samantha is still drinking from te fountain, but is finding it more and more difficult. Through Patricia's encouragement, she manages to take one last sip. She feels the power, and finally believes she really is talking with her adoptive mother. Patricia apologizes to Samantha for all the bad things she did to her, but Samantha forgives her. Right after Kara, J'onn, Alura, Imra and Brainy leave, Samantha wakes up, and floats in the air, stunning Lena and Alex. Ruby is also happy and proud of her mother. Samantha asks, where is Supergirl. Lena and Alex tell her she and the others already left, but she goes after them with her new powers.
As the Children of Juru and Reign prepare for another attempt, Reign hears the Legion ship. Kara, J'onn, Mon-El and Alura arrive and a battle begins. Supergirl and Reign fight outside, while others defeat and chain the witches. However, Reign ultimately proves too powerful for Kara, and is about to strangle her, when Samantha arrives, wielding the Sword of Juru. She plunges it to Reign, and Kara then throws her into the hole. But before dying, Reign unleashes a number of superpowered heat visions on others, which causes Mon-El, Alura and Samantha to die. J'onn is also hit, but he survives barely. Heartbroken over the deaths, she laments herself for allowing herself to use lethal force. As she speaks to J'onn, she then recalls how Harun-El split Samantha and Reign, and realizes it can also reunite them. Then she remembers Mon-El's ring, and asks Alex and Lena to find her a time disruption, so she can go back in time, and do it over. She flies into it and sees visions and hears the words of her loved ones from her past. She then reaches the moment just before Samantha struck the sword into Reign. She stops Samantha, knocks Reign on the ground and tells Mon-El to throw her the Harun-El. She catches it, puts it into Reign's hand, and tells Samantha to touch it. All three end up in Juru, where Samantha beats Reign, and forces her to drink from the Fountain of Weakness. With this done, the ghosts of Juru grab Reign, and drag her into the fountain, banishing her forever.
Everyone return to DEO, where Winn has agreed to join the Legion and go to the future. Kara and Mon-El talk and finally they both get a closure about their relationship: they would always love each other, but the duty they have in their own times is more important. Kara also has come to terms, that Earth is her home now, even though being in Argo City was like being in home, but something was missing there. In there, she could be Kara Zor-El, but not Supergirl or Kara Danvers, as it is these three identities that make her who she is. Kara and Mon-El goodbye in good terms, and Mon-El gives her his ring to remember him, just like Kara gave him her necklace. The DEO then say goodbye to Winn, with Alex and Kara crying over his leave, and Kara acknowledges that Winn is a hero. Alura leaves back to Argo City, saying goodbye to her daughter. Lena also gives her the remaining Harun-El and a recipe to make it, to ensure Argo City's survival. Alex goes to see J'onn and is about to resign from the DEO, due to her desire to be a mother, but J'onn denies it, and instead promotes Alex to Director of the DEO. J'onn intends to leave and do what M'yrnn wanted him to do.
The next day, Alex and Lena meet Samantha and reveal that she is now 100% human, with no trace of Kryptonian DNA in her. Samantha thanks them and Alex particularly for being there for Ruby. J'onn puts a hat on and walks in the streets of National City. In a news report, James publicly reveals his identity as Guardian. In Kara's apartment, Kara and Alex eat pizza and discuss everything have happened, with Kara deciding to stay on Earth. Alex wonders are they going to be okay and Kara assures they will be.
Lena is in her lab studying Harun-El with Eve, revealing she didn't give it all away.
Going back 48 hours, as Kara, Reign and Samantha touched the Harun-El, a black mist escaped into the sky without anyone noticing. The mist eventually reaches Siberian border. In a Russian military base, soldiers see someone walking towards the base. As the person enters the light, it is a young woman, a perfect copy of Kara, created by Harun-El.
- The episode was previously titled "Supergirl Returns". This was a callback to the 2006 film Superman Returns.
- This is the first season finale to not feature Calista Flockhart.
- This episode marks the last appearances of Jeremy Jordan, Chris Wood, and Odette Annable as series regulars.
- After stopping a tidal wave, Kara tells Brainy, "You're all clear kid, now let's blow this thing and go home!", quoting Han Solo from the 1977 film Star Wars: Episode IV – A New Hope. Brainy notes it as a film reference, to which Kara is surprised that they haven't showed him any Star Wars films.
- The climax scene in which Reign holds Supergirl in a headlock, nearly killing her, is reminiscent of the scene in the 2013 film Man of Steel, in which Superman snaps General Zod's neck.
- Supergirl using a disturbance to travel back in time and save Mon-El and her mother is a reference to Superman reversing time to save Lois Lane from Superman: The Movie.
- Brainy's evil distant relative is implied to be the original Brainiac, Viril Dox, also known as the Collector of Worlds who is his ancestor and one of Superman's archenemies. In the comics, Viril shrunk and bottled Krypton's capital city, Kandor (which housed Supergirl's family) to add to his collection of planets deemed worthy of conquest.
- Kara is given Mon-El's Legion ring, implying at least honorary membership in The Legion. In the comics, she is a full-fledged member of the Legion of Super-Heroes.
- The appearance of an alternate Kara in Siberia is a homage to the 2003 comic book story Red Son by Mark Millar. This was an alternate version of the Superman origin story in which of Kansas, Kal-El lands in the Soviet Union in 1938 and becomes that country's champion.
- Chyler Leigh revealed in an interview that the scene where Alex says goodbye to Winn was Chyler saying goodbye to Jeremy, and she was actually sobbing.
- Leigh also mentioned that she was so emotional while filming Alex and Winn's goodbye scene that she had to walk away during takes because it was "hard to look at him."
- Despite the recurring appearances of the Legion and their flight rings throughout this season, the fact that Superman keeps a Legion flight ring at the Fortress of Solitude (first seen in Season 1) is never explained.
- The recounting of Persephone's myth is incorrect; she was not condemned to live in the underworld for the rest of her life, but to spend part of the year with her mother Demeter, and the rest with Hades in the underworld. As Demeter controlled the harvest, this myth was used to explain why the autumn and winter months are barren.
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