- "Imagine my excitement when one of my fighters told me about Slaver's Moon. I jumped at the opportunity to help expand its market."
- —Veronica Sinclair
Kara is moved by the story of a missing woman named Izzy and decides to investigate despite Snapper Carr’s order to leave it alone. Kara takes Mon-El with her to the last place Izzy was seen and the duo find themselves thrust through a portal to another planet, Slaver's Moon, where the downtrodden are sold as slaves. The leader of the trafficking ring is none other than Roulette. To make matters worse, Slaver's Moon has a red sun, which means Kara and Mon-El are stripped of their powers and stuck on the planet with no way home. Meanwhile, back on Earth, Alex blames herself for Kara's disappearance, fearing she's become too focused on her new relationship with Maggie and too complacent in watching out for her sister.
Supergirl is chasing diamond robbers escaping in a van. They use surface-to-air missiles on her, but she manages to stop the van. However, she only apprehends the driver. The other two robbers are then caught up by the Guardian. He subdues one of them and the last one runs towards Winn's van. Winn manages to knock him down with the van door, but celebrates too early and the robber beats him up and nearly shoots him until Guardian comes to his aid. Elsewhere, a doctor shows a patient something.
At the DEO, Supergirl confines to Alex her frustration with the Guardian, but Alex knows, she is really frustrated by something else. She then reveals that she is getting bored, since she hasn't been helping people lately. Alex then goes to meet with Maggie.
The next morning Alex is feeling happy for Maggie having stayed the night. At CatCo, Kara and James bring Snapper their stories about last night but he shows no interest. A woman then arrives to talk with Snapper and tells that her daughter, Izzy, is missing. Snapper isn't interested but Kara promises to find her.
Kara goes to the Al's Dive Bar and is surprised to see Mon-El working there. When Mon-El failed to give her club soda, she goes to see Maggie for info about Izzy's disappearance. Maggie tells her that there have been around twenty mysterious disappearances in National City, but none of them seems to have any connection with each other. Maggie gives Kara a flash drive about the disappearances.
Kara arrives at DEO to have Winn look at the flash drive. Winn tries to hide his last night's bruising with sunglasses, and Kara is shocked to see it. Winn says he got mugged. Alex arrives feeling happy, and Kara is happy about it. Winn learns that the disappearances actually have one thing in common. All of them had applied to a blood trial, and disappeared shortly after that.
Kara is leaving CatCo to investigate, when Mon-El arrives. He decides to come with her, and Kara reluctantly agrees. At the blood trial, a doctor takes them in and shows them a portal. The doctor then transforms into an alien and his goons attempt to force Kara and Mon-El through the portal. They manage to beat them but the doctor escapes. Kara tells Mon-El to go to DEO and send a strike team after her. As Kara goes through the portal, she ends up in a different planet, and is attacked. She manages to beat the attacker but it leaves her bleeding. Another alien attacks her, but she is saved by Mon-El, who decided to come help her. Mon-El sees Kara's bleeding and Kara then points towards the planet's red sun, meaning she and Mon-El have no powers here. The portal then closes behind them.
James arrives at the DEO to talk with Winn, but he refuses to speak with him or join for Guardian's next job. Alex arrives and asks if they have seen Kara. Winn says she is after a story. Alex takes a DEO squad to the blood trial and discovers the portal. J'onn recognizes it as a matter transfer portal, a way for an instant interstellar travel.
At the other planet, Kara and Mon-El keep arguing as Mon-El urges them to leave, but Kara is determined to save the kidnapped humans, despite not having powers. Mon-El suddenly steps into a trap and Kara is unable to free him. An alien then appears and they surrender.
At the DEO Winn has managed to discover the portal's destination. J'onn recognizes it as the planet Maaldoria, in the Arcturus system, the heart of intergalactic slave trade. Alex gets worried when J'onn tells her that Arcturus' sun is red sun. Things also goes worse when he also tells that Maaldorian atmosphere is toxic to Martians. Maggie arrives and Alex is unable to be with her for now, due to her worry for Kara.
At Maaldoria, the alien who caught Kara and Mon-El, turns out to be a friendly nomadic Maaldorian named Jo, who surprisingly can talk in English. He reveals that they are in Maaldoria and Mon-El knows the planet, as Daxam used to buy it's slaves from there. Mon-El also insists to Kara that he never agreed with Daxam's slavery. Kara shows Jo Izzy's picture and he reveals she and other humans we're taken to the Red Fort. He also claims there is no way inside, but Kara says there is one: the front door. She and Mon-El go to the fort, pretending to have surrendered.
Kara and Mon-El are taken to the cells, where the humans have been imprisoned. Kara promises to help them even without powers. Kara is then surprised to see Roulette, who reveals her that after she ruined her fighting club, she learned from one of the alien fighters about Maaldoria, and has started a slave trade between Earth and Maaldoria.
At the portal on Earth, Alex is putting together a strike team and orders Winn to come along to make sure the portal stays open on the other side. Winn is scared out of his mind but Alex tells him that she too was terrified during her first time on the field.
Humans are then to be taken away, but Kara puts herself in the line to protect the people. She gets a painful torture, which inspires the people to fight back. They overpower the Maaldorians and Kara and Mon-El forces the doctor and Roulette into the cell. They then start to flee.
Alex, Winn and DEO squad arrive at Maaldoria. Alex leaves Winn behind to keep the portal open. As Kara leads the humans out, Mon-El uses a rifle to hold back pursuers. A tall alien then catches up to them. He stops Maaldorians from pursuing them after seeing Mon-El, bowing to him. The group flees and rendezvous with Alex. They flee back to the portal, where Winn is forced to fight a Maaldorian. He manages to grab a rock and knock out the alien, feeling exhilarated. Kara, Alex and Mon-El arrive with the humans. Humans flee the portal, but Maaldorians arrive with pursuit. Alex then throws a Yellow Sun Grenade J'onn gave her to give Kara a power charge. She manages to get the Maaldorians to retreat and they all escape, along with Jo. Kara destroys the portal controller.
At CatCo, Kara takes Izzy to her mother and then delivers her report about Izzy's disappearance to Snapper. Snapper doesn't look interested, but Kara then explains that she wants to be a reporter, who follows stories she believes in. Snapper takes the article, and as Kara leaves, Snapper smiles to himself and says "'Atta girl, Danvers".
Alex invites Maggie to her place and tries to explain herself, but Maggie reveals that she figured out that Supergirl is Kara, as Kara is the only person that would make Alex worry the way she did. Alex says she is sorry and Maggie gives her one chance to be serious about their relationship.
Mon-El arrives at Kara's apartment with Club Soda, and Mon-El reveals that after today he has decided to become a superhero like Kara. Kara agrees with it as long as he listens to her.
At Maaldoria, the hooded Daxamites arrive to question them about Mon-El's whereabouts. The doctor reveals that he's on Earth. Daxamites then kill him.
- The episode's title is a reference to the unproduced Superman movie Superman Lives. The episode's director, Kevin Smith, wrote the original screenplay for the film.
- Winn says, "You just hit a button and you're thinking with portals", a phrase that was first introduced in 2006 with the teaser trailer for the video game Portal, which ends with the line "now you're thinking with portals.".
- An Earth-38 Dominator can be seen when Supergirl, Mon-El, and the rest of the humans are captured. Kara mentions that she hates the Dominators, indirectly referencing the four-night crossover event, , in which she went to Earth-1 and fought the Dominators of that universe.
- When Winn is left alone at the portal, he says a few times, "I am not a red shirt". In the Star Trek franchise, a "redshirt" is a term used by fans to denote a character wearing a red Starfleet uniform that is killed, typically expendable characters with little significance to the plot besides their death.
- When Kara asks Winn why he is wearing sunglasses, he answers, "The future is so very bright". This might be a reference to the Timbuk3 song "The Future's So Bright, I Gotta Wear Shades".
- Kara wants to watch the TV series The Night Of with Alex.
- Winn compares the transmatter portal to a Stargate, a reference to the 1994 movie of the same name.
- When Alex explains to Maggie that she doesn't want to go to work and that just wants to stay home with her instead, she fake coughs and says, as an excuse, "I've got the black lung!". This line wasn't scripted; it was added later when on set, Chyler Leigh said it as a joke while talking with the cast and crew, who all laughed and loved it. Thinking it would be a good line for the scene, they added it into the episode.
- The line Mon-El says, "Start the car!", was also improvised on set and kept in by the director.
- At the end of the episode, while escaping Slavers' Moon, Winn exits through the portal twice. He can be seen being pushed through the portal by Supergirl at the 32:08 mark, and then standing next to her still on Slavers' Moon at the 32:24 mark and going through it a few seconds later.
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