The episode opens with Supergirl chasing down a van full of jewel theives. One of the thieves shoots a bazooka at her, but Supergirl stops the van with her heat vision. When two of the thieves get away on foot, Guardian arrives to help put them down. Winn tries to assist capturing the last thief, but the guy gets the drop of him and almost puts a bullet in his head until Guardian shows up at the last second.
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Later that night, Supergirl complains to Alex that she’s in “a funk”. Alex offers to cancel her date with Maggie to spend the night with her girlfriend. The next morning, Maggie and Alex decide to play hookie rather than go to work.
Over at CatCo, Jimmy and Kara write dueling stories about the jewel heist from the night before, while Snapper just wants coffee. A woman suddenly shows up at Snapper’s office, asking for help to find her missing daughter. Kara promises to help, but Snapper doesn’t think it’s really news.
Kara shows up at the (still unnamed) alien bar to find that Mon-El is now working there as a bartender. Mon-El tries to offer some unwanted advice about Kara’s “funk”, but Kara is there to meet Maggie for the case file on her missing person. Turns out there’s been a spike of missing person cases recently.
A scientist leads a young man into an empty warehouse and then pushes him into a mystery alien device. The scientist then thanks Roulette for finding “another perfect candidate”.
Kara asks Winn for help about the missing persons and finds out that all of the missing people had bloodwork done. She also notices that Winn has a nasty shiner from the jewel thief.
Kara and Mon-El go undercover to investigate the clinic where all the missing people had their bloodwork taken. The scientist from earlier tries to strongarm the pair into an alien portal, but Supergirl fights them off. Kara runs through the portal to try to find the girl, but the planet on the other hand has a red sun and so she’s powerless. An alien attacks Supergirl, but Mon-El rushes through the portal and saves her.
Supergirl and Mon-El argue over what to do next, but Supergirl is determined to find the missing people…who are probably in a giant fortress in the distance.
Jimmy shows up at the DEO to check on Winn, but Winn is freaking out and tries to quit on Jimmy. Alex shows up and asks if either of them have heard from Kara.
As Mon-El and Kara approach the fortress, Mon-El gets his leg caught in a trap. They end up getting “captured” by an alien named Jo.
Alex finds the portal, which J’onn recognizes as a transmatter portal.
Jo ends up freeing Mon-El and Supergirl and explains that the missing people are being sold as slaves. Mon-El realizes they’re on Slaver’s Moon. Jo says there’s no way into the fortress…but Supergirl points out there’s an easy way to get into the fortress…by surrendering.
The scientist (who’s an alien) leads Supergirl and Mon-El to the human prisoners. Supergirl finds the missing girl, and admits that she doesn’t have her powers right now. Roulette then shows up and says that they should stop meeting like this.
Alex and the DEO track Supergirl to Slaver’s Moon, but J’onn can’t joing the rescue mission because the air’s toxic to Martians. Alex then freaks out on Maggie, because Kara went missing while she was playing hookie.
Roulette tells Supergirl she went into the slave trade because Supergirl dismantled her club. A Dominator shows up and says he’s going to buy all the humans.
Alex recruits Winn to join her on the mission to rescue Kara. Winn tries to back out, but Alex says that no one gets better at handling dangerous situations by running away.
Supergirl gives the prisoners a pep talk, but Mon-El wants to know why she’s always the one trying to save people. Supergirl says that she never gives in, and that’s the difference. When the slavers arrive, Supergirl tries to protect everyone, so the aliens start zapping her with giant tazers. Watching Supergirl getting zapped inspires Mon-El and the other humans to fight back, and they overwhelm the soldiers. After locking Roulette and the alien scientist up, Supergirl leads the humans towards freedom.
Alex leads Winn and a strike team through a portal to Slaver’s Moon. Alex tells Winn to unlock the portal so they can all get home. While Alex attacks the fortress from the front, Supergirl and Mon-El protect the prisoners. Mon-El almost gets shot by one of the guards, but the Dominator stops the guard and bows to Mon-El.
Alex and Supergirl reunite at the edge of the fortress, while Winn beats up a guard trying to stop him. Winn then starts the portal just as Supergirl, Alex, and everyone else arrives. Jo the alien saves Alex from getting shot, and heads to Earth with everyone else. As a warship arrives, Supergirl sees Izzy get dragged away by one of the guards. Alex throws a yellow sun grenade at Supergirl to power her up and rescue Izzy.
After Supergirl saves Izzy and gets through the portal, she heat visions the controls, stopping the portal from getting used ever again.
Kara brings Izzy back to her mother, and then dumps a report about Slaver’s Moon on Snapper’s desk. Although Snapper acts grumpy to Kara’s face, he smiles after she leaves.
Winn tells Jimmy that he’s back in the Guardian game, and brags about his intergalactic exploits.
Alex apologizes to Maggie for going crazy on her earlier, and admits that she felt responsible for Supergirl’s disappearance. Maggie says that she knows that Kara is Supergirl, but isn’t convinced that Alex won’t run again. Alex says that she wants to be happy with Maggie.
Mon-El shows up at Kara’s apartment with club soda and admits he was wrong about not getting involved. He tells Kara that he wants to be a superhero, just like her. Kara gives Mon-El a copy of the Highlights magazine he was reading at the clinic and says that he was more Gallant than Goofus.
The episode ends with the mysterious hooded aliens from last episode arriving on Slaver’s Moon looking for Mon-El. The alien scientist says Mon-El’s on Earth, so the aliens disintigrate him with a pistol.
